change the main color to the red of the (TEAM-)M logo

this has to be done in less/theme.less (https://git.violoncello.ch/websites/theme-melvin/blob/c4f7bca43d771ad48d5b504cc0d0426c1133d591/less/theme.less) because the less files are built/compiled to the different css files and overwrite them (so no changes directly to the css files)
We should maybe even create our own style template in less/styles/ so our changes are independent from the rest of the theme...

@melvin less just extends css with things like variables, what makes it easier to change e.g. a color value in all places in the css... so less isn't that hard to understand, the base is still raw css... however it's a bit hard to get a working node/npm setup to compile less to css, so you can just do the changes in the less files and ask me to compile it, if you want...