We have various colors in the gantt rendering hardcoded, which means that taking
some colors from the theme results in white on white rendering with dark themes.
To avoid this, don't take any colors from the theme for now, making the canvas
rendering simply not honor dark mode instead of being somewhat unusable in it.
The header remains honoring theme colors, as that blends in with the treeview
header and doesn't look too bad with the canvas remaining white background.

Properly supporting dark mode requires a bunch of preparatory work towards
providing our own light and dark theme CSS for the canvas colors, so is left
for the future.

Closes #250