Redmine

Hello,
I surfed around by I did not find any answer for my issue so I hope not to duplicate the request.
I know how to realize a theme where only css and images modifications are done, but I need also to make some html changings (for exammple in base.rhtml file).
The best way I found so far is by making a plugin where I put my modified .rhtml file but this way make the new .rhtml file override the old one for every theme while I need to load my html files only when I load my css theme and get back to the original for the other themes. Is there a way to put the modified .rhtml files inside the theme folder?
Hope the question is clear.
Thank you