• Install Stylus browser extension: Firefox, Chrome
• Go to userstyles.world and search for "Lemmy"
• Pick one and click
• It will open a new tab, where you will have to click one more button.

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.

Preview of some styles

Click on the image to go to style page.