To manage your website styles the easiest way: On your Admin Panel Add/Edit Page, on the Custom Stylesheet menu choose "Default Stylesheet" if you are planning to use your "main" style for it.  This will allow you to easily switch it out with a new style and have it applied to all these pages.   In another words, you can create more Stylesheets with different names, play around with your website styles, and choose to set one to Default, to apply to all of these pages set to use the Default Stylesheet.

If you have a page which you would like to use a special Style for, set that Custom Stylesheet menu on the Add/Edit page for it to that custom stylesheet name.

Admin Panel > Configure Site > Styles Manager goes to your Default Stylesheet.  Go to Styles Manager > List Stylesheets to choose which Stylesheet you want to edit.  User Manual Styles Mamager topics are here.

No coding experience necessary to create your own style!  But, if you do want to extend the software, feel free to post your custom css (styling) coding questions or tips as a new topic in this forum!