Update style, enhance mobile support

wwwroot/css/file-explorer.css

@@ -254,7 +254,7 @@
   color: var(--text) !important;
   border: none !important;
   border-radius: 3px !important;
-  opacity: 0.7;
+  opacity: var(--semi-transparent);
 }
 
 .fe-mde-wrap .editor-toolbar button:hover {
@@ -323,6 +323,6 @@
 }
 
 .fe-save-btn:not(:disabled):hover {
-  opacity: 0.85;
+  opacity: var(--semi-transparent);
 }