Some colors don't support background colors

[ridiculousfish]

set fish_color_autosuggestion 000000 --background FFFFFF

This should show a white background color with autosuggestions, but does not.

[floam]

Also cannot set foreground colors for selected pager items, which is annoying as often the contrast of the colors involved causes terrible readability and need to be changed.

[zanchey]

As noted in #6939, fish_color_comment is also affected.

[shlemas]

I recently wanted to play with making a terminal/fish theme based on flatwhite which is a bit of an odd theme in that it does more with background colors instead of foreground colors, and I ran into this as well. fish_color_command has the same issue, maybe all of these. I believe I traced the cause to this line which hardcodes the background color to rgb_color_t::normal():

fish-shell/src/highlight.cpp

Line 1302 in 30c7a17

outp.set_color(rv.resolve_spec(color, false, vars), rgb_color_t::normal());

I naively changed the second parameter to the following, making the call consistent with another usage I'd seen in the codebase:

rv.resolve_spec(color, true, vars)

The second argument (Boolean) is what causes it to parse out the --background portion of the color spec or not. That ultimately happens in the parse_color function:

fish-shell/src/output.cpp

Line 468 in 30c7a17

rgb_color_t parse_color(const env_var_t &var, bool is_background) {

But nothing changed. It seemingly was still unable to resolve the color I was specifying. That's all I've done so far, no actual debugging or deeper digging yet. I assume the background color is hardcoded to rgb_color_t::normal() for a reason.