how can I check some of my other themes for such injections?
Thanks in advance.
Use TAC (theme authenticy checker) and wordpress theme check plugins.
They tell if there is any encoded PHP in the theme and also how many outgoing links it contains.
If a theme has an injection, both will give you warnings with full details (file and line number) :-)
VirusTotal is often misleading for themes and plugins. It only recognizes un-obfuscated open redirects to known malware sites within the code.
Cheers, Johnny63