Fix profiler seek line color in light modes, update on theme change

[ckaiser]

Fixed the profiler seek line being basically invisible in light modes, also added a call to _update_plot() to force it to redraw when changing themes, since otherwise it'd get stuck with the old colors until there was any activity.

Before:

After:

[Calinou]

Tested locally, it works as expected:

simplescreenrecorder-2024-05-26_20.40.53.mp4

Note that the Visual Profiler doesn't update any of its colors when the theme changes. It might be worth looking into this in a separate PR 🙂

[akien-mga]

Thanks!