I'm pretty sure I did... I did it several times. But just now I tried setting a custom theme for the HBoxContainer I was using (problem still there), filled all the other custom styles with StyleBoxEmpty (problem still there), deleted the custom theme (problem still there), and when I made a new custom theme... the problem went away. Very strange. Godot's GUI skinning options aren't very intuitive, on top of that... It's very confusing. At least it works, I guess. haha