theme: simplify stylesheet for color variants

[McShelby]

Creating color variants currently requires copying a lot of boilerplate styles. Let's get rid of this.

[DanielRuf]

Maybe also interesting, HSL colors are often better for creating variants. You can change the hue value or just the saturation or lighting to change one aspect of a color.

matcornic/hugo-theme-learn#483

[McShelby]

Sounds interesting. I lack knowledge regarding CSS colors. So if you want to work on it, it may be interesting to have a second accent color for coloring the links.