Changed the function to override theme font color from "add_color_override" to "add_theme_color_override", as the previous version is not recognized by my 4.3 Godot Editor.
Judging by the documentation archives, it seems that "add_color_override" was the version used in Godot 3.
From Control's 4.3 documentation, it seems C# functions have also been updated to include Theme in their name. It would make sense, so I updated it too, but I'm not 100% sure it actually works this way.
