Comment bar color themes

[Sjmarf]

Comments have a vertical colored bar on the leading edge. The color of the bar shows the depth of the comment. The colors used for this should be customisable in Appearance settings.

Apollo had this; someone requested this on mlemapp here.

[EricBAndrews]

Per our design tenet of "users shouldn't be able to make the app ugly," we do have to approach this with a certain degree of deliberation--right now we have very nice color alignment because we're sticking to the Apple palette, so just slapping an RGB wheel into settings would more or less undermine that whole paradigm. Curated color palettes are an obvious option, as are color pickers that are restricted to the Apple palette.

[Sjmarf]

Curated color palettes sounds good to me - that's what Apollo had.

[Sjmarf]

Possible child of #252