Theoretical Solution right now:
1.) change the default css path in the web admin forum configuration to your folder
2.) rename your css file AAA to the name of the forum css file BBB
3.) copy/paste all forum css information into your file AAA
4.) remove your css includes in your header (css will now be included by the forum app
5.) now you can make any changes in AAA without having the fear other css files will overwrite this ...
when this solutions is totally

- admin: please delete the post

best regards
sebastian