With Opera, you can do something similar to Firefox add-on called Greasemonkey.

Greasemonkey is used to customize webpage to be displayed as your own preference. However, if you're not Firefox user, you also can do this by using Opera.

How to do that? Let's explore those steps.

Installing Opera

If you haven't had Opera web browser, you can find and download it for free here.

Opera Preferences

Now, open your Opera and then go to ▸ Opera ▸ Preferences... Then you will be brought into Opera Preferences, on the General Section.

Advanced Preferences

Now, go to the Advanced section. On the sidebar you will see some sections. Choose on Content then click on the button labelled JavaScript options...

JavaScript Options

Near to the OK button, there will be a button which is cut off a bit in my version of Opera. Click on it and choose a folder in which you've stored your javascript for customizing Opera's display.