Fix expanded menu when screen width is <1000px for Twenty Twenty theme (

#3790)

* Fix amp-lightbox when screen width is <1000px for Twenty Twenty theme

* Use a more specific selector rather than using `!important` to apply the rule

includes/sanitizers/class-amp-core-theme-sanitizer.php

@@ -1030,6 +1030,12 @@ static function() use ( $args ) {
 						}
 					}
 
+					@media (max-width: 999px) {
+						amp-lightbox.cover-modal.show-modal {
+							display: unset;
+						}
+					}
+
 				}
 				<?php elseif ( 'twentyseventeen' === get_template() ) : ?>
 					/* Show the button*/