Stacks uses the built-in macOS UI colors to shade things. Unfortunately Big Sur basically just removed a bunch of the differentiation – flattening a lot of colors out. Pretty annoying to be honest. A bunch of the subtle window colors are all just white now.
I think some of them are actually very slightly off-white – you can see if you click the desktop, making RW a background window, the stacks toolbar background gets a little darker, but on my monitor they’re close enough to white that it kind of doesn’t matter, you know.
The toolbars are all what macOS calls “Vibrance-Views” (ha! LOL, white is not very “vibrant” if you ask me). It means they’re translucent to the thing behind them. This means that you can see the color of a stack show through the toolbar in an etched-glass sort of effect. In previous macOS versions light-mode vibrant-views had a sort of gray tint to them even with white content behind them. In Big Sur they’re pretty much just plain white.
I’m probably going to make some changes because of this. But I’m not 100% certain where to go from here. There aren’t a lot of ways to customize this sort of translucent view. You only get to choose from a few color modes – and every choice affects both light/dark mode – and how those things will appear in the previous macOS versions. If I can find another variant of the translucent view that looks better in Big Sur and not too terrible in Catalina then I’ll go with it.
As some say, it’s currently looks a bit better in dark mode. Again this is not my purposeful doing. These are all macOS standard window/background/control colors. I’ve stopped customizing colors. Because of Dark-Mode/Light-Mode it’s too challenging to customize colors. It means designing every UI thing with lots of variants.
macOS nerd info: did you know there are two other “modes” for the UI. Yep, “High-Contrast” is also a “mode”. It’s in the Accessibility Settings inside the System Settings.
That mean that there is High-Contrast-Light-Mode and Low-Contrast-Light-Mode and High-Contrast-Dark-Mode and Low-Contrast-Dark-Mode.
So, I stopped using custom colors pretty much everywhere. I just use standard system colors/views.
The only colors that are really custom in Stacks are the green/orange/pink of the Partial/External/Template buttons.