Enforce transparent background on editor toolbar elements (#11569) (#11583)

Co-authored-by: Lauris BH <lauris@nix.lv>
(cherry picked from commit 38c773d8a5)

Co-authored-by: zeripath <art27@cantab.net>

web_src/less/_editor.less

@@ -29,6 +29,10 @@
     opacity: 1 !important;
 }
 
+.editor-toolbar a:not(:hover) {
+    background-color: transparent !important;
+}
+
 .editor-toolbar i.separator {
     border-left: none;
 }