Compromised theme, most likely. Check theme files for inserted code, along with inserted code in other files. Change credentials everywhere (Host, database, wp admin, ftp).
Look at all files manually, plus files that shouldn’t be there (like extra ICO files as in .somefile.ico
, which have php code inside. (Notice the leading dot, which indicates a hidden file, so make sure you are showing hidden files also.) Check your htaccess file also.
For the probably-compromised theme, nuke it and re-upload theme files from known good source.