Style may not apply to the URL of your instance, and in that case, you need to do it yourself.
- Click on the Stylus extension icon, and then click manage.
- New tab will open with a list of all installed styles. Click on the style that you just installed.
- That will open the style editor. You want to look for a field that looks like this and click on the + button.
- That will add a new field just below it. In the empty field, type the URL of your instance.
- In the top left corner of the page, there is a Save button. Click that, and you are done.