if you are using Windows, you can use notepad or wordpad to modify css or php files (i reccommend wordpad)
if you are using Mac, you can use the textedit, or NovoEdit
. I'm a mac user, i'd prefer to NovoEdit.
So, maybe you can do these steps:
1. Open the files (imagemenu.css) from its folder in Windows. I mean, you don't need to open it through Filezilla. Just extract the zip file of the slidingdoor theme that you downloaded to your computer, then open the files.
2. Edit the lines, with any program you'd prefer. Then save it.
3. Upload (and overwrite) it to the corresponding folder in your webhost through the FTP software.
4. Test your web
And btw, yes, you're getting closer to solve the problems