create a variable vBgColor, you can assign a valur like: RGB(255,255,255)
use this variable to assign the background color: $(vBgColor) or just =vBgColor
In sheet properties create a trigger for activate sheet with an action, Type: external, Action: establish variable, then assign to the varibale vBgColor the color you want to change, ie: RGB(180,180,255)
I'm not sure what you mean with "Activation of sheet"? If a sheet isn't activated, it's either completely invisible, or hidden by another sheet so that only the sheet tab is visible. In the first case, changing the background color is rather useless. In the second case, you can only change the tab color (see Sheet properties->General->Tab settings->custom colors).
To change a tab color when a sheet is moved to the front, you can use a color expression like: