Welcome to the forum! 
It seems we may have updated our mint-themes package a little too preemptively since Arch hasn’t updated most of the Cinnamon packages yet. Probably when Linux Mint announces the 20.3 release (currently in beta), they will be updated. For now, you can try mint-themes-legacy from the AUR (Arch User Repository).
What? Who? Please check your spelling and don’t invent abbreviations. The distro is called Manjaro and there is no abbreviation.
Please see: