fix for #351 on 10.14+

[dustinknopoff]

Summary

It looks like the main issue in #351 was the use of NSAppearance.vibrantDark instead of .darkAqua. This is due to vibrantDark using color blending instead of the system colors. Source This was probably done because .darkAqua only appears in 10.14+. I guess the option here is keep having too dark buttons prior to 10.14 or just always use the unified_titlebar colors.

Related Issues

Related to #351

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