diff --git a/plugins/default_colors/default_colors.css.template b/plugins/default_colors/default_colors.css.template
index 4b269ba..87e22a0 100644
--- a/plugins/default_colors/default_colors.css.template
+++ b/plugins/default_colors/default_colors.css.template
@@ -1,3 +1,3 @@
 :root {
-  %s
+%s
 }
diff --git a/plugins/default_colors/default_colors.php b/plugins/default_colors/default_colors.php
index b898814..1928cc9 100644
--- a/plugins/default_colors/default_colors.php
+++ b/plugins/default_colors/default_colors.php
@@ -6,6 +6,7 @@
  * Allow users to easily overrides colors of the default theme.
  */
 
+use Shaarli\Config\ConfigManager;
 use Shaarli\Plugin\PluginManager;
 
 const DEFAULT_COLORS_PLACEHOLDERS = [
@@ -15,7 +16,32 @@ const DEFAULT_COLORS_PLACEHOLDERS = [
 ];
 
 /**
- * When plugin parameters are saved
+ * Display an error if the plugin is active a no color is configured.
+ *
+ * @param $conf ConfigManager instance
+ *
+ * @return array|null The errors array or null of there is none.
+ */
+function default_colors_init($conf)
+{
+    $params = '';
+    foreach (DEFAULT_COLORS_PLACEHOLDERS as $placeholder) {
+        $params .= trim($conf->get('plugins.'. $placeholder, ''));
+    }
+
+    if (empty($params)) {
+        $error = t('Default colors plugin error: '.
+            'This plugin is active and no custom color is configured.');
+        return array($error);
+    }
+}
+
+/**
+ * When plugin parameters are saved, we regenerate the custom CSS file with provided settings.
+ *
+ * @param array $data $_POST array
+ *
+ * @return array Updated $_POST array
  */
 function hook_default_colors_save_plugin_parameters($data)
 {
@@ -27,16 +53,20 @@ function hook_default_colors_save_plugin_parameters($data)
             ? default_colors_format_css_rule($data, $rule) .';'. PHP_EOL
             : '';
     }
-    file_put_contents($file, sprintf($template, $content));
+
+    if (! empty($content)) {
+        file_put_contents($file, sprintf($template, $content));
+    }
+
     return $data;
 }
 
 /**
- * When linklist is displayed, include isso CSS file.
+ * When linklist is displayed, include default_colors CSS file.
  *
  * @param array $data - header data.
  *
- * @return mixed - header data with isso CSS file added.
+ * @return mixed - header data with default_colors CSS file added.
  */
 function hook_default_colors_render_includes($data)
 {
@@ -48,6 +78,26 @@ function hook_default_colors_render_includes($data)
     return $data;
 }
 
+/**
+ * Create a valid CSS rule from parameters settings and plugin parameter.
+ *
+ * @param array  $data      $_POST array
+ * @param string $parameter Plugin parameter name
+ *
+ * @return string CSS rules for the provided parameter and its matching value.
+ */
+function default_colors_format_css_rule($data, $parameter)
+{
+    if (empty($data[$parameter])) {
+        return '';
+    }
+
+    $key = str_replace('DEFAULT_COLORS_', '', $parameter);
+    $key = str_replace('_', '-', strtolower($key)) .'-color';
+    return '  --'. $key .': '. $data[$parameter];
+}
+
+
 /**
  * This function is never called, but contains translation calls for GNU gettext extraction.
  */
@@ -59,10 +109,3 @@ function default_colors_translation()
     t('Background color (light grey)');
     t('Dark main color (e.g. visited links)');
 }
-
-function default_colors_format_css_rule($data, $parameter)
-{
-    $key = str_replace('DEFAULT_COLORS_', '', $parameter);
-    $key = str_replace('_', '-', strtolower($key)) .'-color';
-    return '  --'. $key .': '. $data[$parameter];
-}
diff --git a/tests/plugins/PluginDefaultColorsTest.php b/tests/plugins/PluginDefaultColorsTest.php
new file mode 100644
index 0000000..b9951cc
--- /dev/null
+++ b/tests/plugins/PluginDefaultColorsTest.php
@@ -0,0 +1,195 @@
+<?php
+
+namespace Shaarli\Plugin\DefaultColors;
+
+use DateTime;
+use PHPUnit\Framework\TestCase;
+use Shaarli\Bookmark\LinkDB;
+use Shaarli\Config\ConfigManager;
+use Shaarli\Plugin\PluginManager;
+
+require_once 'plugins/default_colors/default_colors.php';
+
+/**
+ * Class PluginDefaultColorsTest
+ *
+ * Test the DefaultColors plugin (allowing to override default template colors).
+ */
+class PluginDefaultColorsTest extends TestCase
+{
+    /**
+     * Reset plugin path
+     */
+    public function setUp()
+    {
+        PluginManager::$PLUGINS_PATH = 'sandbox';
+        mkdir(PluginManager::$PLUGINS_PATH . '/default_colors/');
+        copy(
+            'plugins/default_colors/default_colors.css.template',
+            PluginManager::$PLUGINS_PATH . '/default_colors/default_colors.css.template'
+        );
+    }
+
+    /**
+     * Remove sandbox files and folder
+     */
+    public function tearDown()
+    {
+        if (file_exists('sandbox/default_colors/default_colors.css.template')) {
+            unlink('sandbox/default_colors/default_colors.css.template');
+        }
+
+        if (file_exists('sandbox/default_colors/default_colors.css')) {
+            unlink('sandbox/default_colors/default_colors.css');
+        }
+
+        if (is_dir('sandbox/default_colors')) {
+            rmdir('sandbox/default_colors');
+        }
+    }
+
+    /**
+     * Test DefaultColors init without errors.
+     */
+    public function testDefaultColorsInitNoError()
+    {
+        $conf = new ConfigManager('');
+        $conf->set('plugins.DEFAULT_COLORS_BACKGROUND', 'value');
+        $errors = default_colors_init($conf);
+        $this->assertEmpty($errors);
+    }
+
+    /**
+     * Test DefaultColors init with errors.
+     */
+    public function testDefaultColorsInitError()
+    {
+        $conf = new ConfigManager('');
+        $errors = default_colors_init($conf);
+        $this->assertNotEmpty($errors);
+    }
+
+    /**
+     * Test the save plugin parameters hook with all colors specified.
+     */
+    public function testSavePluginParametersAll()
+    {
+        $post = [
+            'other1' => true,
+            'DEFAULT_COLORS_MAIN' => 'blue',
+            'DEFAULT_COLORS_BACKGROUND' => 'pink',
+            'other2' => ['yep'],
+            'DEFAULT_COLORS_DARK_MAIN' => 'green',
+        ];
+
+        hook_default_colors_save_plugin_parameters($post);
+        $this->assertFileExists($file = 'sandbox/default_colors/default_colors.css');
+        $content = file_get_contents($file);
+        $expected = ':root {
+  --main-color: blue;
+  --background-color: pink;
+  --dark-main-color: green;
+
+}
+';
+        $this->assertEquals($expected, $content);
+    }
+
+    /**
+     * Test the save plugin parameters hook with only one color specified.
+     */
+    public function testSavePluginParametersSingle()
+    {
+        $post = [
+            'other1' => true,
+            'DEFAULT_COLORS_BACKGROUND' => 'pink',
+            'other2' => ['yep'],
+            'DEFAULT_COLORS_DARK_MAIN' => '',
+        ];
+
+        hook_default_colors_save_plugin_parameters($post);
+        $this->assertFileExists($file = 'sandbox/default_colors/default_colors.css');
+        $content = file_get_contents($file);
+        $expected = ':root {
+  --background-color: pink;
+
+}
+';
+        $this->assertEquals($expected, $content);
+    }
+
+    /**
+     * Test the save plugin parameters hook with no color specified.
+     */
+    public function testSavePluginParametersNone()
+    {
+        hook_default_colors_save_plugin_parameters([]);
+        $this->assertFileNotExists($file = 'sandbox/default_colors/default_colors.css');
+    }
+
+    /**
+     * Make sure that the CSS is properly included by the include hook.
+     */
+    public function testIncludeWithFile()
+    {
+        $data = [
+            'css_files' => ['file1'],
+            'js_files' => ['file2'],
+        ];
+        touch($file = 'sandbox/default_colors/default_colors.css');
+        $processedData = hook_default_colors_render_includes($data);
+
+        $this->assertCount(2, $processedData['css_files']);
+        $this->assertEquals($file, $processedData['css_files'][1]);
+        $this->assertCount(1, $processedData['js_files']);
+    }
+
+    /**
+     * Make sure that the CSS is not included by the include hook if the CSS file does not exist.
+     */
+    public function testIncludeWithoutFile()
+    {
+        $data = [
+            'css_files' => ['file1'],
+            'js_files' => ['file2'],
+        ];
+        $processedData = hook_default_colors_render_includes($data);
+
+        $this->assertEquals($data, $processedData);
+    }
+
+    /**
+     * Test helper function which generates CSS rules with valid input.
+     */
+    public function testFormatCssRuleValid()
+    {
+        $data = [
+            'other1' => true,
+            'DEFAULT_COLORS_BLIP_BLOP' => 'shinyColor',
+            'other2' => ['yep'],
+        ];
+        $result = default_colors_format_css_rule($data, 'DEFAULT_COLORS_BLIP_BLOP');
+        $this->assertEquals('  --blip-blop-color: shinyColor', $result);
+
+        $data = ['unknown-parameter' => true];
+        $result = default_colors_format_css_rule($data, 'unknown-parameter');
+        $this->assertEquals('  --unknown-parameter-color: 1', $result);
+    }
+
+    /**
+     * Test helper function which generates CSS rules with invalid input.
+     */
+    public function testFormatCssRuleInvalid()
+    {
+        $result = default_colors_format_css_rule([], 'DEFAULT_COLORS_BLIP_BLOP');
+        $this->assertEmpty($result);
+
+        $data = [
+            'other1' => true,
+            'DEFAULT_COLORS_BLIP_BLOP' => 'shinyColor',
+            'other2' => ['yep'],
+        ];
+        $result = default_colors_format_css_rule($data, '');
+        $this->assertEmpty($result);
+    }
+}