How Do I Add Common Styles to All Pages?

You can add CSS styles for the whole site in your project settings. To do this, click on the "Arrow" icon next to your project name and select Settings. Then go to the "Custom Code" tab, click on the "+" icon, and select <style>. Screenshot:

After that you will need to give any name to the code that you place and then set the desired CSS values without the tag. Here, for example, I changed the background of all my pages: