That could just be his system’s accent color. Users can get that on current macOS versions in System Prefs.
BUT you can change the accent color of your app for Big Sur. If the user has the new “Multicolor” accent color chosen.
You will need Xcode. 12 would be better, but 11 will work well enough. Use File -> New -> File and choose to create an Asset Catalog. I recommend keeping your catalog, so save it somewhere important and not just the desktop. Inside the catalog, press the “+” at the bottom left and create a new color set. By default, your color set will have two vales: Any Appearance and Dark Appearance. If you show the attributes inspector using View -> Inspectors -> Attributes, you can change the number of colors in the set with the Appearances menu. My advice is to set colors for both any and dark. Also, give your color set a name.
Now that you have your assets catalog, you need to compile it.
/Applications/Xcode-beta.app/Contents/Developer/usr/bin/actool --compile DESTINATION_PATH --minimum-deployment-target 11.0 --platform macosx CATALOG_PATH. I’ll admit I don’t actually know the right minimum deployment target, but 11 has worked for me.
This will create an
Assets.car file in your
DESTINATION_PATH. Oh right, that means
DESTINATION_PATH should be a folder, not a file. Now you’ll need to use Build Automation to have the
Assets.car folder added to your app’s Resources folder. Lastly, you need to add a key to your Info.plist. Add
NSAppAccentColorName as a string with the value set to the name of the color set in your asset catalog.
Done. Now on Big Sur, your app will adopt the colors if the user has the Multicolor accent color selected.
I should probably put this in a blog post, but that might wait until Big Sur is closer to shipping.
Edit: Oh yeah that’s definitely just the system accent color, since the left is Catalina. And sorry I missed the CSS question. No, you cannot get the system accent color. There is a CSS property for it, but Apple has it always return blue to prevent advertisers from using it as part of a visitor signature.