There is css/default.css which is delivered with Mantis.
In config file I found variable "$g_css_include_file" to use other css file.
First thought was "Cool, I can use default.css and use $g_css_include_file to append my css code from separate file".
But behaviour is different because when I use $g_css_include_file then only this file is used and default.css is ignored.
I verified this statement by checking html header.
So my question is "How to add my own css code without touching default.css?".
I didn't find the right solution from the Internet.