Black and white dithered graphics are the default when you open Decker and many people choose to work within that limitation, but the program does support 16-color palettes and there's been several published color projects.
If you'd like to see a couple of examples from last December's Deck Jam, these two are in color in two different styles:
Fear and Loneliness in Latona's Void and Desker
But also here are two black-and-white games from the same event that have a clear visual style that I think is approachable with-or-without any knowledge of Decker as a program.
The Family Record and Broken Gate (It's retro b&w but very well done)
I don't know if this helps! 🙂But if you have any other specific questions about what it's possible to do visually in the program, I'm sure someone can answer.