mol-plugin: reverted msp-layout-region background !important, because it broke the visual style

src/mol-plugin/skin/base/layout/common.scss

@@ -13,7 +13,7 @@
 
 .msp-layout-region {
     overflow: hidden;
-    background: $default-background !important;
+    // background: $default-background;
 }
 
 .msp-layout-static, .msp-layout-scrollable {