They've always had white backgrounds for me. But then that's not the stock lockscreen you are using but rather one made by CM. Locker, which is not related in any way how the phone will behave.
And DO NOT use anything made by CM. They not only steal your data to sell to people like advertisers and hackers, none of their so called booster apps actually do work and slow the phone down.
As I said, you only have that option to turn them transparent because you aren't using the lockscreen of the phone, but rather a different third party app.Weird. I've managed to set it to have transparent backgrounds for the longest time until it was updated just a few minutes ago.
And thanks for your advice. Would you perhaps have any other applications that can help me change the default lock screen that Android has? It is actually quite ugly.