How Do I Add Common Styles to a Page?

You can add CSS styles for a specific page in the page settings. To do this, right-click the page 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 a name to the code that you're placing and then set the desired CSS values without the <style> tags. Here, for example, I changed the background of my page: