Since v13 the drive buttons are rendered with an ugly outline. I tried to reset the toolbar (which has been ported from v12) with no luck.
Opus 12 did exactly the same thing.
The letter overlays only appear by default if the drive-buttons have labels turned off (since otherwise most drives would have the same icon and nothing to tell them apart). They use the toolbar's text color, and outline it with either black or white for contrast.
You can get rid of the letters by either turning on labels for the button (so the drive letters appear as normal button labels next to the icon rather than overlaid in the corner of it) or add
iconlettersoff to the command to suppress them while keeping the labels turned off, e.g.:
Adding some info from another thread about this:
The letters use the button or toolbar's text color, so you can change it if you wish.
The way they are drawn hasn't actually changed. The default text color on the Location Bar toolbar may have changed, and Dark Mode also affects other toolbars (of course).