How to reskin game GUI in Unity

Go to Assets -> Resources -> Popups

Choose the UI that you want to modify. For example, let's select the Settings UI

Look in the Inspector

Select Menus_label Source Image

Change the color to the one you want

Repeat the process for other images, buttons, and text elements

Use the eyedropper tool (pipette) to pick a color from any image or reference

To change the background, reskin the main-background.png file. You can find it in the Sprites -> general-ui folder. The background is a gradient, so to reskin it, you'll need to use a graphic editor.