Various CSS tweaks (#21244)

- Remove arc-green specific rules and instead fix the colors in the base
rules.
- Make file table row border visible on arc-green.
- Remove remnants of fomantic accordeon module that was removed.

web_src/less/_form.less

@@ -134,7 +134,7 @@ textarea:focus,
 
 .form {
   .help {
-    color: #999999;
+    color: var(--color-secondary-dark-5);
     padding-bottom: .6em;
     display: inline-block;
   }