Try the built-in appearance first
It needs no extra tool and is easy to undo in Settings. If you only want to change brightness, code colors, or fonts, stop there.
When Dream Skin helps
Dream Skin adds image backgrounds and translucent panels. It is a separate tool. Its theme ZIP contains theme.json and artwork, which Codex cannot import by itself.
How to choose
Preview a theme first. Install Dream Skin if you want the image background. Otherwise, keep using the built-in appearance.