Media: Make image editor help icon scheme-aware.

The image editor help toggle icon used a hardcoded classic blue color. Replace it with a CSS custom property so the icon follows the user's admin color scheme.

Props dervishov, huzaifaalmesbah, jamesbregenzer, mukesh27, ozgursar, wildworks.
Fixes #64937.

git-svn-id: https://develop.svn.wordpress.org/trunk@62481 602fd350-edb4-49c9-b593-d223f7449a82

Author: t-hamano

src/wp-admin/css/media.css

@@ -1187,7 +1187,7 @@ border color while dragging a file over the uploader drop area */
 	margin: -1px 0 0 -1px;
 	padding: 0;
 	background: transparent;
-	color: #2271b1;
+	color: var(--wp-admin-theme-color);
 	font-size: 20px;
 	line-height: 1;
 	cursor: pointer;
@@ -1196,9 +1196,9 @@ border color while dragging a file over the uploader drop area */
 }
 
 .image-editor .imgedit-settings .imgedit-help-toggle:focus {
-	color: #2271b1;
-	border-color: #2271b1;
-	box-shadow: 0 0 0 1px #2271b1;
+	color: var(--wp-admin-theme-color);
+	border-color: var(--wp-admin-theme-color);
+	box-shadow: 0 0 0 1px var(--wp-admin-theme-color);
 	/* Only visible in Windows High Contrast mode */
 	outline: 2px solid transparent;
 }