FR: simplify overriding of default colors

[noahmayr]

Is your feature request related to a problem? Please describe.
When wanting to override a color from cli/src/config/colors.toml, e.g. setting the description color to blue whenever there is no description (description placeholder) regardless of whether the commit is the working copy or if it's empty you would have to add the following to your config:

[colors]
"description placeholder" = "blue"
"empty description placeholder" = "blue"
"working_copy description placeholder" = "blue"
"working_copy empty description placeholder" = "blue"

Describe the solution you'd like
Since I've heavily customized my log colors, something like ui.use_builtin_colors = false would work very well for me.

Describe alternatives you've considered
Alternatively a flag like !important from CSS that allows overriding the default style from the example above with just the following would be ok as well:

[colors]
"description placeholder" = { fg ="blue", override = true }

[yuja]

ui.use_builtin_colors = false

It could be ui.color-theme = <theme-name> (and [color-themes.default], etc.), but I don't know if we'll need this kind of indirection.

[noahmayr]

That's interesting, it would actually not only solve this issue, but allow us to ship 2 or 3 default themes which each highlight the information provided differently.

1. Default (basically what we have now)
2. Minimal (same as default with only the most important fields colorized and less important fields dimmed)
3. Contextual (something similar to what I'm using, which colorizes most fields of a change in the same color and gives a unique color to working copy changes, immutable changes, changes with conflicts, etc.)