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/fomantic/theme.config.less

@@ -50,7 +50,6 @@
 @table      : 'default';
 
 /* Modules */
-@accordion  : 'default';
 @calendar   : 'default';
 @checkbox   : 'default';
 @dimmer     : 'default';