Fix background color

[micahmo]

Pull Request Description

Alrighty I finally took the time to look into some of the color issues. As much as I wanted to clean up my old PRs before moving on to new stuff, I think these issues need to be fixed before we can release a new stable version. I also finally built the last version of the app prior to the color changes, so I can easily reference back and forth. (If anyone else is interested in doing the same, you can use commit 0e148d1. You will also have to downgrade your local Flutter version to 3.19.x. And for some reason I also had to upgrade share_plus to ^10.0.0.)

I am starting by tackling getBackgroundColor, which is a function we use quite a bit throughout the app to generate a nice "background color" in various circumstances. It was previously based on theme.dividerColor, but either that has changed, or the way darken and lighten work has changed. To maintain consistency, I have decided to just hard-code the colors to what they used to be. It's not really based on the theme anyway, so I don't think it's a big deal.

One of the easiest places to see the color in use is behind instance taglines, so that's what I'll use for my screenshots, but it is used elsewhere throughout the app as well. Note that the difference in dark/black mode is very subtle, but I used a color picker to make sure it's exactly the same now.

Issue Being Fixed

Issue Number: #1587 (comment)

Screenshots / Recordings

	Light	Dark	Pure Black
Old Version
Before
After

Checklist

• If a new package was added, did you ensure it uses an appropriate license and is actively maintained?
• Did you use localized strings (and added appropriate descriptions) where applicable?
• Did you add semanticLabels where applicable for accessibility?

[hjiangsu]

LGTM! Just curious, but how does it look on Material You themes?

[micahmo]

how does it look on Material You themes?

Seems ok!