Add dark mode support on macOS Mojave and later

This issue was migrated from bug 15511 in our old bug tracker.

Original bug information:

Reporter: rbr…@‍abe…
Status: IN_PROGRESS
Product: Wireshark
Component: Build process
OS: macOS 10.14
Platform: All
Version: 2.6.1

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information