If your browser is already set to Arabic language you can just head over to http://addons.opera.com to see the changes, otherwise you can check it out here.
Øyvind Håkestad, our user interface guru, says of the RTL support:
While in the process of making addons.opera.com work for Arabic, the easy way out would have been to let only the text flow from right to left. But we wanted to do it properly, by making the whole page rtl.
This involved converting all of our Cascading Style Sheets (CSS) from ltr (left to right) to rtl. Since we have a fair amount of CSS code (that keep changing all the time), we figured doing this manually would be too much work. It would also be error prone. We already had an automated process for making CSS from Less code, and we thought plugging in something that converted our CSS from ltr to rtl would be cool.
There are already code out there doing this type of conversion, but for various reasons, we chose to implement our own. It is written as a library for Node, and it can be installed through npm. We've made the code available on Github, and we hope it may be useful for other people as well :-)
You can find it here: https://github.com/operasoftware/flipcss