Move all CSS color attributes to variables

[maxxcrawford]

          Are we using `white` as a standard, or `#ffffff`? I don't mind either, just want to keep things consistent.

Originally posted by @codemist in #3030 (comment)

[pdehaan]

PROTIP: Lots of fun grep and research in the linked comment thread from @maxxcrawford's original post.

Also, here's a few low-hanging places we're hardcoding colors:

git grep -Ein "#[A-F0-9]{3,8}" src/**/*.css | cat

src/client/css/global.css:111:  box-shadow: 0 6px 6px -8px #0000;
src/client/css/global.css:118:  box-shadow: 0 6px 6px -8px #000;
src/client/css/global.css:162:  box-shadow: 0 -6px 6px -8px #000;
src/client/css/global.css:235:  box-shadow: inset 0 0 64px #80808011;
src/client/css/nav.css:15:  border-image: linear-gradient(#0000 3%, var(--gray-10) 5%, var(--gray-10) 95%, #0000 97%) 1;
src/client/css/partials/breaches.css:200:  /* #99bff2 is var(--blue-50) with an alpha value of 0.4 */
src/client/css/partials/breaches.css:201:  background-color: #99bff2;
src/client/css/partials/landing.css:155:  background: linear-gradient(74deg, #e53fb0 15%, #a260f7 121%);
src/client/css/partials/landing.css:245:  background: linear-gradient(74deg, #e53fb0 15%, #a260f7 121%);
src/client/css/variables.css:51:  --monitor-gradient: linear-gradient(-90deg, #ff9100, #f10366 50%, #6173ff);
# ... // cut all the existing css color variable definitions // ...