Allow for user defined css file. Add 'file_exist' check when building…
… all.css file to avoid error if no user defined css file provided. Make git ignore user-defined.css file
In my opinion, instead of hiding the file in public/css/user-defined.css, it should be "user.css" or "config.css" or something like that in the main directory where the other user-editable files such as config.ini live.
Thanks for the pr. I think regarding some other individual css wishes it makes sense to add this. I realized it in another way with the idea to make font.css also optional.
Thank you very much for being dedicated to selfoss!!!