Dark mode is betrer for battery life in general. Dark mode makes everything black or gray Themes change icons, call screen sometimes, the menu The battery is adaptive meaning it learns your usage
Here are some more thoughts to help you get started. One of the first things I do with a new phone is uninstall or disable apps that came with the phone that I have no intention of using. Themes are purchased in the Samsung Theme Store. You will need to sign in with your Samsung account username and password. Most themes do not work with Dark Mode because it overrides the theme wallpaper and turns it black. Keyboard is affected too. If your phone has a battery protector built in, you can use it to stop your phone at 85% so it doesn't overcharge. This is best if you charge overnight. Color palette is part of the Material You theming and is separate from the Samsung Theme Store. You would pick a wallpaper and then the color palette comes up with several choices based on your wallpaper. All phone batteries degrade over time. Another thing you want to watch for is keeping your storage less full. You need to allow some breathing room for the phone to do its processes. If it is close to full, that will slow down your phone. Clear your cache occasionally for apps that you use a lot. You will find it in Storage for each app under Settings. There are some good articles on the Internet that will give you good advice.
I'd go to the Galaxy Store and download the full Good Lock suite of apps and just have fun customizing the phone. And from the Google Play store download a couple of launchers. Everything is better than the stock Samsung launcher. Find one you like
