I recently did an install of an SMF (or Simple Machines Forum) for another site. I wanted to keep the basic stock theme, but change some the color scheme to match the website. After performing a search on Google and Bing, I couldn’t find any options other than downloading a new Theme. It wasn’t hard to find the file I was looking for with a little digging around in the Administration Center. From The Administration Center you will want to look for the ‘Themes and Layout’ link. Click this. Now look for the ‘Modify Themes‘ option. From here there is a link to ‘Browse the templates and files in this theme‘. At this point you will see a file system similar to using an ftp. Open the CSS folder and find the index.css file.
Main > Administration Center > Modify Themes > Browse the templates and files in this theme > css > index.css