diff --git a/projects/plugins/jetpack/changelog/add-css-vars-to-csstidy b/projects/plugins/jetpack/changelog/add-css-vars-to-csstidy
new file mode 100644
index 000000000000..25bc845a6619
--- /dev/null
+++ b/projects/plugins/jetpack/changelog/add-css-vars-to-csstidy
@@ -0,0 +1,5 @@
+Significance: patch
+Type: compat
+Comment: 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 ecb5f72931c5..ecae3063e5aa 100644
--- a/projects/plugins/jetpack/modules/custom-css/csstidy/class.csstidy.php
+++ b/projects/plugins/jetpack/modules/custom-css/csstidy/class.csstidy.php
@@ -279,6 +279,7 @@ function __construct() {
 		$this->settings['preserve_css'] = false;
 		$this->settings['timestamp'] = false;
 		$this->settings['template'] = ''; // say that propertie exist
+		$this->settings['preserve_css_variables']     = false;
 		$this->set_cfg('template','default'); // call load_template
 		$this->optimise = new csstidy_optimise($this);
 
@@ -1178,7 +1179,21 @@ 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 1.0
+	 */
+	public function property_is_css_variable( $property ) {
+		return preg_match( '/^--([a-zA-Z0-9]+[\-_]?)+(?<![\-_])$/', $property );
 	}
 
 	/**
diff --git a/projects/plugins/jetpack/modules/custom-css/custom-css.php b/projects/plugins/jetpack/modules/custom-css/custom-css.php
index d821c77fef3d..17b809a5c1cc 100644
--- a/projects/plugins/jetpack/modules/custom-css/custom-css.php
+++ b/projects/plugins/jetpack/modules/custom-css/custom-css.php
@@ -1623,9 +1623,10 @@ static function filter_attr( $css, $element = 'div' ) {
 		$csstidy->set_cfg( 'remove_last_;', false );
 		$csstidy->set_cfg( 'css_level', 'CSS3.0' );
 
-		// Turn off css shorthands when in block editor context as it breaks block validation.
+		// Turn off css shorthands, and allow css variables, when in block editor context otherwise block validation is broken.
 		if ( true === isset( $_REQUEST['_gutenberg_nonce'] ) && wp_verify_nonce( $_REQUEST['_gutenberg_nonce'], 'gutenberg_request' ) ) { // phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotSanitized, WordPress.Security.ValidatedSanitizedInput.MissingUnslash
 			$csstidy->set_cfg( 'optimise_shorthands', 0 );
+			$csstidy->set_cfg( 'preserve_css_variables', true );
 		}
 
 		$css = preg_replace( '/\\\\([0-9a-fA-F]{4})/', '\\\\\\\\$1', $css );
diff --git a/projects/plugins/jetpack/phpunit.xml.dist b/projects/plugins/jetpack/phpunit.xml.dist
index 10aa1e98f164..3cd084bd3b63 100644
--- a/projects/plugins/jetpack/phpunit.xml.dist
+++ b/projects/plugins/jetpack/phpunit.xml.dist
@@ -128,6 +128,9 @@
 		<testsuite name="seo-tools">
 			<directory prefix="test" suffix=".php">tests/php/modules/seo-tools</directory>
 		</testsuite>
+		<testsuite name="csstidy">
+			<directory prefix="test" suffix=".php">tests/php/modules/csstidy</directory>
+		</testsuite>
 	</testsuites>
 	<groups>
 		<exclude>
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
new file mode 100644
index 000000000000..8c0321179420
--- /dev/null
+++ b/projects/plugins/jetpack/tests/php/modules/csstidy/test-class.jetpack-csstidy.php
@@ -0,0 +1,67 @@
+<?php
+/**
+ * Class WP_Test_Jetpack_CSSTidy.
+ *
+ * @package automattic/jetpack
+ */
+
+require_jetpack_file( 'modules/custom-css/csstidy/class.csstidy.php' );
+
+/**
+ * Class WP_Test_Jetpack_CSSTidy
+ */
+class WP_Test_Jetpack_CSSTidy extends WP_UnitTestCase {
+
+	/**
+	 * The tested instance.
+	 *
+	 * @var csstidy
+	 */
+	public $instance;
+
+	/**
+	 * Sets up each test.
+	 *
+	 * @inheritDoc
+	 */
+	public function setUp() {
+		parent::setUp();
+		$this->instance = new csstidy();
+		$this->instance->set_cfg( 'optimise_shorthands', 0 );
+		$this->instance->set_cfg( 'discard_invalid_properties', true );
+	}
+
+	/** Provides values for CSS custom property patterns */
+	public function custom_property_matches_provider() {
+		// phpcs:ignore Squiz.PHP.CommentedOutCode.Found -- false positive
+		// 'test case description' => [ 'input', 'expected output', 'preserve_css_variables' ].
+		return array(
+			'test_removes_css_var_properties_by_default' => array( 'div {--base-color:red;color:var(--base-color)}', "div {\ncolor:var(--base-color)\n}", false ),
+			'test_css_var_properties_preserved'          => array( 'div {--base-color_for_1st-child:red;color:var(--base-color)}', "div {\n--base-color_for_1st-child:red;\ncolor:var(--base-color)\n}", true ),
+			'test_css_var_properties_with_no_alphanum_chars_removed' => array( 'div {--_:red;color:var(--base-color)}', "div {\ncolor:var(--base-color)\n}", true ),
+			'test_css_var_properties_ending_in_hyphen_removed' => array( 'div {--base-color-:red;color:var(--base-color)}', "div {\ncolor:var(--base-color)\n}", true ),
+			'test_css_var_properties_ending_in_underscore_removed' => array( 'div {--base-color_:red;color:var(--base-color)}', "div {\ncolor:var(--base-color)\n}", true ),
+			'test_unknown_properties_removed'            => array( 'div {clowns-nose:red;color:var(--base-color)}', "div {\ncolor:var(--base-color)\n}", true ),
+			'test_invalid_css_properties_removed'        => array( 'div {--$//2343--3423:red;color:var(--$//2343--3423)}', "div {\ncolor:var(--$//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( 'preserve_css_variables', $preserve_css_variables );
+		$this->instance->parse( $input );
+		$this->assertEquals(
+			$expected_output,
+			$this->instance->print->plain()
+		);
+	}
+}