css: improve RGB hex color and length matching

[hochhaus]

• Allow RGB hexadecimal colors in multiple formats per the spec. For example, all of the following formats are valid.

  fff

  ffffff

  ffffffff

  https://developer.mozilla.org/en-US/docs/Web/CSS/color_value#rgb_colors

• Allow decimal lengths without a leading zero. For example, the all of the following formats are valid.

  .5em 0.5em 0 1em

  https://developer.mozilla.org/en-US/docs/Web/CSS/length#syntax

[buro9]

I'm happy to merge this when the regexp's are made public (I see no reason that they're not public and I default public - see the ones in /helpers.go which are default public.

[buro9]

I'm going to merge and rename things 👍

[hochhaus]

Thanks.