Search for Roblox at the bottom. The extension will now give you several theme options for Roblox. Click on the theme you like and click on Install Style. Now, go to Roblox and click on the Stylish ...

If you did everything correctly, you should be able to see your character detached from the green background. Now, toggle off the ‘Show Canvas’. Also, make sure ‘Transparent canvas’ is enabled here ...