Fix code-expansion arc-green theme bug (#10180) (#10185)

* fix code-expansion theme bug

* working solution without important

* no new color

* the midle

web_src/less/themes/arc-green.less

@@ -1111,6 +1111,9 @@ input {
 .lines-code .hljs {
     background-color: #2a2e3a !important;
 }
+td.blob-excerpt {
+    background-color: rgba(0, 0, 0, 0.15);
+}
 
 .code-view .active {
     background: #554a00;