The 3rd solution solved my issue, thank you very much!
Today is the first time I've had to set up a Linux distro to my liking. I installed the XFCE Debian 64bit PeppermintOS released on March 4th and wanted to use dark mode because that's my preference. I go to the Appearence section of the Settings and change it to "Marwaita Dark Peppermint" because I like the way it looks, but I noticed it doesn't set apps to dark mode along with the system. Is there a way to change this? I see that the style "Adwaita-dark" does change apps to dark mode so I know it...