Twenty Fifteen: use (and calculate) hex colors instead of opacity/rgba

[celloexpressions]

Using opacity and rgba colors for text colors to facilitate secondary colors and colorschemes is a bit hacky, and results in inconsistent results. It also makes it difficult to understand the color patterns of the theme at a first glance, which is important for anyone looking to make customizations or a child theme. We can just as easily calculate the alternate colors dynamically when generating the colorschemes, and cache the resulting css rather than running it on every load.

I have a patch for this in my head, although I probably won't have time to write it out for a couple days. I'm also back and forth on whether it's worth making this change given our timeline, but I think it'll make the theme easier to work with and let us clean up some code.

[celloexpressions]

Also see #30166.

[celloexpressions]

We're not going to have enough time to consider this unfortunately. I'm still not a huge fan of the way this is implemented, but it works fine for users, which is the most important part.