Reorganize Chroma styles (#13934)

This moves the Chroma styles into separate files with base,light and
dark variants. Should work exactly as before.

web_src/less/_base.less

@@ -1416,6 +1416,7 @@ a.ui.basic.label:hover {
 }
 
 .lines-code {
+  background-color: var(--color-code-bg);
   padding-left: 5px;
 }