Dark mode improvements

[calc84maniac]

Improve dark mode support in the following ways:

• Support dark theme on Windows Qt6 by switching to Fusion style if applicable
• Improve detection of the current theme in an OS-independent way
• Detect system theme changes dynamically and update colors to match
• Improve dark mode color contrast in various areas of the UI
• Switch to a syntax highlighter for the disassembly view rather than hardcoding HTML, making it easier to style

I'm going ahead and putting up a PR to generate builds for feedback, and to get some testing on MacOS and Linux since I've only been able to test on Windows. In the meantime I may want to look into improving the Fusion style slightly, because some aspects of it seem a little off layout-wise.

[adriweb]

I tested both Qt5 and Qt6 macOS builds, and so far it's good:

[adriweb]

More tests:

• On Qt6: looks perfect on the Qt6 version. It adjusts automatically when the system pref changes.
• On Qt5: it is fine after you do a run/stop cycle, for the disasm to be showing with the new colors. So it's really not a big problem.