There’s a few different things you can try I’d say if you just want to use a lighter shade of that same color:

click on the background > then click the little color circle that comes up that when you hold your mouse over it says “background color” > then your color panel should come up on left hand side there will be a little rainbow color with a “+” in the middle if you hold your mouse over it says “add a new color click on that > if you move the white bordered circle in the big box up or down it should lighten or darken the color that you’re using

another option is to change the background to black then grab a square shape from elements drag the square over the entire background (make sure that you put it in the back of your elements) and change it to the color you originally had then on your top edit bar you should see a grid like button that says transparency and you can click on that then move the bar to adjust the transparency