Open questions are whether to support header-color settings or remove that feature in favor of T4214, which should be fully completed if so.

If you are interested in modifying these locally, the variables are here: https://secure.phabricator.com/diffusion/P/browse/master/src/applications/celerity/CelerityResourceTransformer.php;78438951c804e3ec184a839d8f96741146a21dd4$172

There are a few things here than come to mind.

Yes gray is terrible for accessibility and gray on gray should be avoided at all cost (and for some reason designers love this :) ). (Dark gray on (near) white however, can very much increase readability enjoyment compared to black on white)

Our base colors are all client side variables, and aren't (for the most part) stored in the CSS. I'd rather build a stylesheet generator for installs that need to provide alternate states, like high contrast, larger fonts, etc, than to try to solve these problems all as one offs. Looking that the links you provided, #444 is the "lightest" grey we could be able to use in any design, so we'd have to give up quite a bit. If your install has needs for this feature, please follow T4213.

It's an accessibility issue:
http://webaim.org/resources/contrastchecker/

Foreground color: #2980b9
Background color: #daeaf3
Contrast Ratio: 3.49:1

CC @scp for his expert accessibility opinion

If there is a way to get this for free that I haven't been able to find, let me know.

There would be a few concerns here:

This is the expected UI, we've been simplifying and flattening the button states and colors. The outline style is used in more auxiliary/optional elements vs. solid for submits. These will fit more into the upcoming full redesign.

Was added and deployed.

Nice, I like that.

I think we're in pretty good shape here now, some minor possible followups in T7858 and elsewhere.

Some followup in T7841 and elsewhere but this is substantially functional.

D12441 actually implements this beautiful, elegant control.