How can I disable the theme you downloaded?

In Firefox:

You have full control over the themes, layouts ans skins.
All themes you download are in the control panel themes in your browser.
To access it click on the plugin, which is at the bottom of the browser on your right. The plugin icon is an S in a box.
Opens a tab with all the themes you downloaded.
Note that the topic you are viewing is marked on the side.
If you want to change the subject just clear you are using and select what you want to view.
If you want to disable the themes just leave them all unchecked that Orkut gets the look originally known. And every time you want to see again some topic that has already downloaded just to access the back panel and mark it.

Google Chrome:

 
Google Chrome: to control the issues in Chrome is different ... why firefox ends up being more practical ...

you have to go there in the chrome tool that sits on top of the browser (control google chrome, in some symbol of a tool) . It will open a tab and you click Extensions.
There you have the option to control the plugin. You can uninstall or disable. You can activate the theme or deactivate.

I hope this helps.