Make preset values !important

Given how the style system works, !important is a good choice
for preset classes the user attaches to the post content.
If those values need to be overridden, in addition to regular CSS,
we can offer hooks for the theme.json processing,
so plugins have an alternate mechanism to modify this behavior.

lib/class-wp-theme-json.php

@@ -791,7 +791,7 @@ private static function compute_preset_classes( $settings, $selector ) {
 						array(
 							array(
 								'name'  => $class['property_name'],
-								'value' => $value[ $preset['value_key'] ],
+								'value' => $value[ $preset['value_key'] ] . ' !important',
 							),
 						)
 					);