Tap your picture at the top left, then Settings. Under Appearance, tap Theme and pick a color theme, or tap Custom there to choose your own color. Each theme has a color for Light Mode and one for Dark Mode, and the app uses the one that matches your device. No Theme uses the plain system colors.
Tap App Icon to choose a different icon for your Home Screen. To change the first day of the week, use Starting Weekday under General.
Default and No Theme are free. The other themes and your own colors are part of Premium — you can tap any theme to try it first. The other app icons are part of Premium too.