In terms of details to make the UI look less plain;
Well, there is a use of several programs (Specifically outside of Roblox as stated) You can also check out png’s online as there are many and implement it into the UI as a detail to use in the UI’s you make, In this, I used the digital particles in the background.
Particles can also be made by using a brush in art software, Then to make it repeat across the screen you can just use the Tile ScaleType. There is also the use of UIGradients with a variation of transparency & colors, and 9Slicing.