Theme colour overrides

[ghalse]

The #db0100 colour is hard-coded in several places in the SCSS, making it quite difficult for theme developers to override.

Doing something like:

:root {
  --simplesamlphp-color: #db0100;
}

and then referencing it elsewhere as e.g.:

.pure-button-red {
  background-color: var(--simplesamlphp-color);
  color: #fff;
}

would make it much easier to override the colour consistently by simply declaring an additional <style> element in _head.twig with the new --simplesamlphp-color variable definition. The same could be done with the other colours that are used.

I'd do this as a pull request, but I think this repo has complexity I do not understand :-)