Scope monaco related overrides only to white theme

Put all monaco related color overrides under .theme-white selector, so
that the colors are non overridden in other themes.

app/assets/stylesheets/page_bundles/_ide_monaco_overrides.scss

@@ -69,8 +69,16 @@
       display: none !important;
     }
   }
+}
+
+.multi-file-editor-holder {
+  height: 100%;
+  min-height: 0; // firefox fix
+}
 
-  .monaco-diff-editor.vs {
+// Apply theme related overrides only to the white theme
+.theme-white .blob-editor-container {
+  .monaco-diff-editor {
     .editor.modified {
       box-shadow: none;
     }
@@ -131,12 +139,9 @@
   }
 }
 
-.multi-file-editor-holder {
-  height: 100%;
-  min-height: 0; // firefox fix
-
-  &.is-readonly .vs,
-  .vs .editor.original {
+.theme-white .multi-file-editor-holder {
+  &.is-readonly,
+  .editor.original {
     .monaco-editor,
     .monaco-editor-background,
     .monaco-editor .inputarea.ime-input {