Improve AutoListCtrl legibility in dark mode

[fruitchewy]

WxWidgets in theory respects system-level dark modes, and most of the components behave in a sane way by default. listmix.ListRowHighlighter however mixes SYS_COLOUR_LISTBOX with SYS_COLOUR_HIGHLIGHT, which has a very different effect with dark system colors resulting in half of the rows being illegible with the intended white font. This change manually overrides ListRowHighlighter's highlight colour with a lightened SYS_COLOUR_LISTBOX when in dark mode.

Before:

After:

[DarkFenX]

Thanks