@Override
	public GTheme getTheme(String themeName) {
		Optional<GTheme> first =
			getAllThemes().stream().filter(t -> t.getName().equals(themeName)).findFirst();
		return first.orElse(null);
	}
Removes the given theme from the set of themes known by this class.  
<p>
Note: this method assumes that there can only exist one theme by a given name in the system.
The equals() method of the GTheme is based on more than just the name, so we cannot use that
to remove the theme from this class.

@param t the theme to remove
Ghidra/Framework/Gui/src/main/java/generic/theme/ApplicationThemeManager.java