feat: add support for macOS accent color

[volskaya]

Adds an option to appearances to highlight ThumbnailView background with macOS accent color instead of the default black, while they're using the macOS theme.

In the picture it's using the hidden iMac purple accent that everyone can unlock with:
defaults write -g NSColorSimulateHardwareAccent -bool YES
defaults write -g NSColorSimulatedHardwareEnclosureNumber -int 7

[lwouis]

Hi @volskaya,

This is an interesting idea. Thank you for sharing a PR.

People have been asking to customize highlight color for a long time. Here's my opinion on it.

I'm afraid I can't merge this PR for a few reasons:

• When changing the accent color, the default command+tab macOS app-switcher doesn't change its highlight color. AltTab is always trying to stay close to the default app-switcher, so it feels native, and almost looks like Apple made it.
• A user may want to use the accent color, but they also want another color. Letting the user pick a color from a color picker here seems like a more open approach. People could pick what they like. See my comments above on limitations of that approach.
• It adds yet another toggle box in the preferences. We are trying currently to reduce their number (see Simplifying the preferences UI #351).

I'm sorry to not merge the PR. I hope you understand the contraints we're in.

Thank you 🙇‍♂️