CSS Overriding

Hi All,

I've made a theme based on a Photoshopped visual (or seven) and it works.  I downloaded an example theme and set to recreating all of the various nested folders for (mostly) one css file per view. It works and the output site looks as it should but with huge amounts of redundant CSS code.
I thought I'd try to clean it all up and attempt to extend the CSS in the core instead of recreating it all.  This works fine for the core CSS file allowing me to have a smallish additional file with just my overrides but I can't accomplish the same trick with any of the other CSS files defined within other plugins.
An example perhaps:
In /mod/mytheme/start.php I've got

This calls /mod/mytheme/views/default/mytheme/css.php in addition to /views/default/css.php which is what I want.
Now I want to do the same thing for /mod/custom_index/views/default/custom_index/css.php but my /mod/mytheme/views/default/custom_index/css.php totally overrides the original.

Is it possible to extend plugin files or do we have to copy and paste like I did originally?
The answer implied from others themes is the copy and paste approach which seems to me to create an overly onerous route for 'mere' visual changes when using 3rd party plugins.

Any help appreciated.  TIA

Beginning Developers

Beginning Developers

This space is for newcomers, who wish to build a new plugin or to customize an existing one to their liking