@Southcoast
To have access to the admin area to edit the theme for a side project/exploratory learning, you will need to utilize the open-source version of Canvas LMS or pay for a licensed Canvas set-up (probably not desired when you are experimenting). Unfortunately, the free version does not provide any admin access to users.
I have run into difficulty getting the open-source version of Canvas LMS set-up before too, but it does look like they updated their documentation recently so perhaps it is easier now or maybe someone else from the Community has tips on getting started with setting it up.
Nonetheless, if you are just looking to experiment with modifying the CSS, you could try looking into making userscripts or a browser extension instead. These would allow you to put your code in a separate file and then modify the interface for yourself while you access the website. This way you can still experiment without the need for admin access and without the concern of impacting the experience for other users. There are several examples of userscripts people from the Community have shared if you search for them. @paul_fynn shared a link to Canvancements, which is a popular set of userscripts that were developed by @James. The code for the Canvas LMS Mods (Basic) browser extension is also open-source and can be viewed on GitHub: Creating a Chrome Extension to Use with Canvas LMS