How to change Mission Control window highlighting
There is not an explicit Mission Control picker for the highlight, nor a slider to increase the highlight size.

However, if you go System Preferences >> Accessibility >> Display, then you will find a number of adjustments that can help you to distinguish the display elements.

The Increase contrast is a good one because it makes everything a little "sharper."

You can further increase the Display contrast, though a little goes a long way.

Increasing the Cursor size might also help, since you'll have an easier time spotting which window is beneath the cursor.

If this still doesn't help, then I'd recommend submitting a feature request to Apple.