The short answer is...you can't...at least not using any of the tools available in Canvas. This is a commonly asked question, though. There is an existing Feature Idea that I would point you to called: Dark Theme for Canvas - Canvas Community. As you can see, there's quite a bit of interest in wanting this capability in Canvas. If you feel that this Feature Idea aligns with something that you would like to see implemented by Instructure (the company that makes Canvas), then I would suggest you give that Feature Idea a star rating and also consider posting a comment to it letting Instructure engineers know why this would be important to you. Here's a Guide which explains the star rating system a bit more: How do idea conversations work in the Canvas Commu... - Canvas Community.
Sorry that I didn't have better news for you, but I hope this has helped to answer your question in some way. Let Community members know if you have any other questions about this...thanks! Take care, stay safe, and be well.