Aw, if you are redirecting to the maintenance folder, chances are that you’re changing the folder level of the request which is upsetting the relative links which are looking relative to the requested URI. I’d tell others to read the tutorial but, since I brought you onboard, either use the <base> tag (with the actual URI) OR links relative to the original request (not the maintenance folder) OR use absolute internal links. It would be easiest for your single file to use the <base> tag but the absolute internal links are awfully handy, too.
Your code will NOT redirect .jpg, .jpeg,.png, .gif, .css or .ico as specified by your second (active) RewriteCond. Therefore, there must be conflicting code elsewhere in your .htaccess so I have to request that you either post it here or PM me if you need to keep it private.
Whoops, remove the leading / in your first (active) RewriteCond as that requires Apache 1.x. If that doesn’t resolve your issue, then Go Back one paragraph.