I'm fairly new to Silverstripe and only recently discovered that you don't edit template files that come with modules, but copy the ones you need to your default template path and edit the copies there. Am I right in thinking this does not apply to CSS files?

Today doing new install of image gallery I moved the default /image_gallery/css/ImageGallery.css to themes/mytheme/css/ but this does not appear to work.

Clarification appreciated please

Thanks

the override system only applies to .ss files

The override system does apply to CSS files if the link to the file is using the themedCSS wrapper rather then an absolute path. Eg in the forum we have Requirements::themedCSS('Forum'); this will look for a Forum.css in your theme path then default to the standard Forum.css.

Just installed image_gallery with a new project (currently using r390 with SS 2.3.7) and noticed that moving ImageGallery.css to local theme CSS folder still does not work. Can your confirm that this is something still not supported and if so if I can manually achieve was suggested before by willr?

The change has been in the 2.4 branch for a while, but I'm not updating the 2.3 branch anymore. It's just too much maintenance for one guy.