In Big Sur, applications can have an accent color to match the branding of the app: Notes is yellow, Music is red, Podcasts is purple, etc. Glyphs is using the standard blue color. The following line changes that to green:
Glyphs doesn’t seem to use the standard blue color, but instead “just” follows whatever your system accent color is. So you could also change that. I guess then all the controls have the same chosen color.
I didn’t look into this, but the defaults command seems to overwrite the system settings. I saw in the Big Sur announcements that an app even can define its own accent color. Didn’t like to overwrite the users choice.
The leftmost accent color (the rainbow) is the default accent color when setting up a new machine. It allows each app to express its own color or use blue by default.
Here, you can specify the red/green/blue components of the color, each in the range [0, 1]. Again, if you don’t like it you can undo the highlight color change like so: