Lock screen wallpaper/clock

[alex_ncfc]

Bit of an odd request this one but it is bugging me so hoping someone can help.

When I set a theme, the lock screen and clock is kind of a solid colour and you can see the wallpaper through it, like so:

But when I change my wallpaper, it loses this effect:

I'm sure I used to know a way to change the wallpaper and keep the "see through" effect, but I can't for the life of me remember how! Any ideas? Obviously not a major issue, or not even an issue lol. Like I said it's just bugging me.