Add a new color palette and rework all site colors (both light mode and dark mode) to
use the new palette.
This ensures that colors are used consistently, from a carefully designed color palette,
instead of being chosen at random.
Before, colors in light mode were chosen on an ad-hoc basis, which resulted in a lot of
random colors and inconsistent design.
The new palette has 7 hues: red, orange, yellow, green, blue, azure (a lighter blue), and
purple. There's also a greyscale. Each hue has 10 shades of brightness, which (including
grey) gives us 80 total colors.
Colors are named like this:
var(--red-0); /* very light red */
var(--red-2); /* light red */
var(--red-5); /* medium red */
var(--red-7); /* dark red */
var(--red-9); /* very dark red */
var(--green-7); /* dark green */
var(--blue-5); /* medium blue */
var(--purple-3); /* light purple */
/* etc */
The color palette is designed to meet the following criteria:
* To have close equivalents to the main colors used in the old color scheme,
especially tag colors, so that changes to major colors are minimized.
* To produce a set of colors that can be used as as main text colors, as background
colors, and as accent colors, both in light mode and dark mode.
* To ensure that colors at the same brightness level have the same perceived brightness.
Green-4, blue-4, red-4, purple-4, etc should all have the same brightness and contrast
ratios. This way colors look balanced. This is actually a difficult problem, because human
color perception is non-linear, so you can't just scale brightness values linearly.
There's a color palette test page at https://danbooru.donmai/static/colors
Notable changes to colors in light mode:
* Username colors are the same as tag colors.
* Copyright tags are a deeper purple.
* Builders are a deeper purple (fixes#4626).
* Moderators are green.
* Gold users are orange.
* Parent borders are a darker green.
* Child borders are a darker orange.
* Unsaved notes have a thicker red border.
* Selected notes have a thicker blue (not green) border.
* Style <button> and <input> elements explicitly instead of relying on browser defaults.
* Style jquery ui buttons the same way as regular buttons.
* Add color variables so we can restyle form inputs in dark mode.
