That is all. The theme will get applied to your Firefox app.
Keep in mind that you can see the themes that you have installed under "Add-ons."
Tap the theme on the "Add-ons," and you can see two options:
Disable - Deactivate the theme.
Uninstall - Delete the theme.
You can deactivate or delete the theme at any time you want.
Remember when you disable or remove the active theme, then the default theme will be applied.