gantt: Improve legibility in dark mode by not using theme colors

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 (closed)