Fix faulty css in example

[DeNelo]

The example showing how to add custom colors was missing a selector.

[squidfunk]

Thanks!

[iBug]

Doesn't seem necessary to me. The theme is already using :root selector so any custom settings also using the same selector should override the theme's variables, assuming custom CSS is always loaded after theme's defaults.

[squidfunk]

Unfortunately, it's necessary due to issues with specificity. I haven't found another way to restructure the CSS to support CSS variable customization and different color schemes at the same time, but if there is, I'm all yours!

[iBug]

Ah, got it. The point here is, elements have [data-md-color-primary=whatever] which has the same specificity as :root, and consequently (as a "direct match" selector) overrides those inherited from :root.