I think the name tells a lot about extension's functionality. Before this extension you had to create new file for every site where you wanted to have User CSS. Thanks to my extension you can just click on a button, write the css rules and press "save" - that's all! You can also use "Live preview" that shows the effect of styling immediately during writing the CSS rules.
- Grey background of textarea means, that it's not possible to edit the User CSS (sites loaded before you installed this extension, speeddial, etc...). If the background is grey on regular site, try to reload the site.
- Extensions might not work on sites with dynamically created iframes
- Even with "Live preview" turned on, you have to use "save" button for permanent changes!!