what I am talking about is like css classes on websites. what I want to know is there a way that I can replicate that in Roblox so if I want to change something like the background color I don’t have to change that on every single button manually and the buttons just fetch the properties of something else.
No, unless you have a Gui library that you can use to create elements and then just reference some colour schema variables. If you do not have a library that creates elements then you can still have a central area where you keep UI colours but you will have to programmatically determine what type of element the GuiObject is and then modify its colours accordingly.