thank you for creating and sharing your ISPConfig theme.
Just a small recemendation, new themes should never overwrite the existing default theme as you theme will get overwritten on updates then. Instead of overwriting files in the default folder, make a new theme by creating a new folder in the themes folder. Inside that folder, just put the file templates/main.tpl.htm from default theme and your new css files. Then change the file templates/main.tpl.htm of your theme so that it loads your css files instead of the ones from default folder.
You can now select your theme in the user settings in ispconfig.