At this time, Canvas does not have a dark mode.
There is an idea discussion for a dark mode/theme for Canvas in the Community: https://community.canvaslms.com/t5/Idea-Conversations/Dark-Theme-for-Canvas/idi-p/364079
If you log into your TWU Canvas account ( https://twu.instructure.com/ ) you should be able to rate the idea and participate in the conversation.
Canvas does accept idea and feature suggestions from all Canvas users. The idea gets opened for voting by other members of the Canvas community and may end up on their roadmap for future development. If you are interested in learning more about this process, you will want to refer to the Canvas guides on idea conversations: https://community.canvaslms.com/t5/Community/How-do-idea-conversations-work-in-the-Canvas-Community/ta-p/2980