[BUG] StackOverflowException when using OnThemeChanged to update the UI

[wolframhaussig]

Describe the bug
when using ThemeRegistry.OnThemeChanged to update the UI, a StackOverflowException is thrown. The reason: ThemeRegistry currently calls the event handlers BEFORE the new theme is set. When the event handler tries to access the current Theme, the check is done again and as the current theme has not been updated yet, the events are triggered again.

Solution: Call handlers AFTER the new theme is set.

To Reproduce

private void ThemeRegistry_OnThemeChanged(object sender, EventArgs e)
{
     ThemeRegistryHolder.ThemeRegistry.CurrentTheme.Apply(this);
}

Expected behavior
No exception should be thrown