Dark mode support

[ysulyma]

Basic dark mode support:

numberscope.dark.mode.mov

• I replaced --ns-color-white with --ns-color-bg when used as a background color, and ns-color-black to --ns-color-fg when used as a text color. Other uses of these variables (e.g. borders) remain intact

• Uses the light-dark() function. This has 87% browser support compared to 96% support for prefers-color-scheme, but has the advantage of colocating the variants; if desired, I can change it to use prefers-color-scheme instead.

• the SVG text also changes color

• I didn't provide a dark variant for --ns-color-primary since it's the brand color and I don't know if it needs to align with other things

[gwhitney]

Not a full review yet, but this feature looks reasonable. One preliminary question: what does light-dark() do when it's not supported?

[ysulyma]

@gwhitney good catch! I could do

--ns-color-fg: #2f2f2f;
--ns-color-fg: light-dark(#2f2f2f, #ffffff);

for backwards compatibility, but I decided to just go with prefers-color-scheme for better compatibility. (There are only five values here so colocation isn't as much of an issue)