diff --git a/projects/plugins/jetpack/changelog/add-custom-css-variable-support b/projects/plugins/jetpack/changelog/add-custom-css-variable-support
new file mode 100644
index 0000000000000..5ae8aa78d0aae
--- /dev/null
+++ b/projects/plugins/jetpack/changelog/add-custom-css-variable-support
@@ -0,0 +1,4 @@
+Significance: patch
+Type: compat
+
+Modified csstidy to allow css variable properties
diff --git a/projects/plugins/jetpack/modules/custom-css/csstidy/class.csstidy.php b/projects/plugins/jetpack/modules/custom-css/csstidy/class.csstidy.php
index 106a770b8bb74..2411526f83ac9 100644
--- a/projects/plugins/jetpack/modules/custom-css/csstidy/class.csstidy.php
+++ b/projects/plugins/jetpack/modules/custom-css/csstidy/class.csstidy.php
@@ -277,6 +277,7 @@ function __construct() {
 		$this->settings['discard_invalid_properties'] = false;
 		$this->settings['css_level'] = 'CSS2.1';
 		$this->settings['preserve_css'] = false;
+		$this->settings['preserve_css_variables']     = false;
 		$this->settings['timestamp'] = false;
 		$this->settings['template'] = ''; // say that propertie exist
 		$this->set_cfg('template','default'); // call load_template
@@ -1180,9 +1181,21 @@ function property_is_valid($property) {
 		$property = strtolower($property);
 		if (in_array(trim($property), $GLOBALS['csstidy']['multiple_properties'])) $property = trim($property);
 		$all_properties = & $GLOBALS['csstidy']['all_properties'];
-		return (isset($all_properties[$property]) && strpos($all_properties[$property], strtoupper($this->get_cfg('css_level'))) !== false );
+		return ( ( isset( $all_properties[ $property ] ) && strpos( $all_properties[ $property ], strtoupper( $this->get_cfg( 'css_level' ) ) ) !== false ) || ( $this->get_cfg( 'preserve_css_variables' ) && $this->property_is_css_variable( $property ) ) );
 	}
 
+	/**
+	 * Checks if a property is a css variable
+	 * Valid patterns must start with `--` and use alphanumeric characters optionally separated by `-`, `--`, or `_`. They must not end with a `-`, `--`, or `_`.
+	 *
+	 * @param string $property The property name to be checked.
+	 * @return bool;
+	 * @access public
+	 * @version 1.5
+	 */
+	public function property_is_css_variable( $property ) {
+		return preg_match( '/^(--[a-zA-Z0-9]+)(([-]{1,2}|[_]?)[a-zA-Z0-9]+)*$/', $property );
+	}
 	/**
 	 * Accepts a list of strings (e.g., the argument to format() in a @font-face src property)
 	 * and returns a list of the strings.  Converts things like:
diff --git a/projects/plugins/jetpack/modules/custom-css/custom-css.php b/projects/plugins/jetpack/modules/custom-css/custom-css.php
index a62e2fe703f8e..4d6e28ea3285b 100644
--- a/projects/plugins/jetpack/modules/custom-css/custom-css.php
+++ b/projects/plugins/jetpack/modules/custom-css/custom-css.php
@@ -221,6 +221,7 @@ public static function save( $args = array() ) {
 		$csstidy->set_cfg( 'discard_invalid_properties', true );
 		$csstidy->set_cfg( 'css_level',                  'CSS3.0' );
 		$csstidy->set_cfg( 'preserve_css',               true );
+		$csstidy->set_cfg( 'preserve_css_variables', true );
 		$csstidy->set_cfg( 'template',                   dirname( __FILE__ ) . '/csstidy/wordpress-standard.tpl' );
 
 		$css = $orig = $args['css'];
diff --git a/projects/plugins/jetpack/tests/php/modules/csstidy/test-class.jetpack-csstidy.php b/projects/plugins/jetpack/tests/php/modules/csstidy/test-class.jetpack-csstidy.php
index a30b3bcc4d528..d8cc9c90fd531 100644
--- a/projects/plugins/jetpack/tests/php/modules/csstidy/test-class.jetpack-csstidy.php
+++ b/projects/plugins/jetpack/tests/php/modules/csstidy/test-class.jetpack-csstidy.php
@@ -61,4 +61,39 @@ public function test_preserve_leading_zeros( $input, $expected_output, $preserve
 			$this->instance->print->plain()
 		);
 	}
+
+	/** Provides values for CSS custom property patterns */
+	public function custom_property_matches_provider() {
+		// phpcs:ignore Squiz.PHP.CommentedOutCode.Found -- false positive
+		// 'test case description' => [ 'input', 'expected output', 'preserve_css_variables' ].
+		return array(
+			'test_removes_css_var_properties_by_default' => array( 'div {--base-color:red;color:var(--base-color)}', "div {\ncolor:var(--base-color)\n}", false ),
+			'test_css_var_properties_preserved'          => array( 'div {--base-color_for_1st-child:red;color:var(--base-color)}', "div {\n--base-color_for_1st-child:red;\ncolor:var(--base-color)\n}", true ),
+			'test_css_var_properties_with_no_alphanum_chars_removed' => array( 'div {--_:red;color:var(--base-color)}', "div {\ncolor:var(--base-color)\n}", true ),
+			'test_css_var_properties_ending_in_hyphen_removed' => array( 'div {--base-color-:red;color:var(--base-color)}', "div {\ncolor:var(--base-color)\n}", true ),
+			'test_css_var_properties_ending_in_underscore_removed' => array( 'div {--base-color_:red;color:var(--base-color)}', "div {\ncolor:var(--base-color)\n}", true ),
+			'test_unknown_properties_removed'            => array( 'div {clowns-nose:red;color:var(--base-color)}', "div {\ncolor:var(--base-color)\n}", true ),
+			'test_invalid_css_properties_removed'        => array( 'div {--$//2343--3423:red;color:var(--$//2343--3423)}', "div {\n--2343--3423:red;\ncolor:var(--$//2343--3423)\n}", true ),
+			'test_broken_or_dangerous_css_removed'       => array( 'div {xss-trap-be-careful:red;}</style><script>alert(\'Gotcha!\')</script>color:var(--base-color)}', '', true ),
+		);
+	}
+
+	/**
+	 * Test that css variable properties are valid/invalid.
+	 *
+	 * @dataProvider custom_property_matches_provider
+	 *
+	 * @param string $input                  potential CSS custom property.
+	 * @param string $expected_output        what we expect css tidy to output.
+	 * @param bool   $preserve_css_variables the value of preserve_css_variables in csstidy's config.
+	 */
+	public function test_custom_property_patterns( $input, $expected_output, $preserve_css_variables ) {
+		$this->instance->set_cfg( 'discard_invalid_properties', true );
+		$this->instance->set_cfg( 'preserve_css_variables', $preserve_css_variables );
+		$this->instance->parse( $input );
+		$this->assertEquals(
+			$expected_output,
+			$this->instance->print->plain()
+		);
+	}
 }