The guide above was written as a guide, but does not walk you through every scenario. I know this because I have written most of it.
That being said, Mac is right in that repairing the theme is likely not going to solve the issue.
That being said, if the issue is in your theme, you're in luck. It's a very lazy hack.
Often case, when it's type of infection we don't find much in the way of backdoors, but we do often see a common thread for the vector being via wp-admin.
So the good news there is by resetting all your passwords, restoring your theme, applying some form of better access controls, you will go a long way.
The other thing I'd look at is if you have the Theme Editor enabled in your Apperance menu. If you do, consider disabling it, as it's often a very common method of injection when we find it in the theme files.
Hope this helps.
Tony