I am guessing you mean the menu bar as shown below
Short answer is that color depends on windows sys colors, which you can change in windows themes, then switch support on in SumatraPDF.
The same applies for menu bar fonts, that’s a windows thing. See how SumatraPDF is forced to use exactly the same system colors and menu font as Notepad.There are registry hacks to change those but you most likely skew up your system (I have just to prove this point).
Or you can use windows magnifier if you need accessibility features, however I find over time I know which word is which and finding the thin bar is best, for easy reading, I switch the magnifier off.
So what can you change inside SumatraPDF ? Well
you can change the fonts in the body of e-books or size of text in other text books and you can skin your own Toolbar as seen here or gradientcolor the main canvas or tint the page within it.
However that all detracts from the main purpose of having a screen full of document with the minimum of clutter.