Hi Andru,
Thanks for reaching out and sharing the feedback. It does sound like a great feature request and I will share the feedback with the Engineering team and you can do that too by posting the request on our UserVoice Page(https://adobexd.uservoice.com/forums/353007-adobe-xd-feature-requests) to include this option in Responsive resize the panel.
Regarding Dark theme, we have already received a similar request on our UserVoice page and I'll request you if you can share your comments and upvote this request: https://adobexd.uservoice.com/forums/353007-adobe-xd-feature-requests/suggestions/12940362-dark-interface-overall-ui-including-side-panels
This is the best way of communicating with the Engineering and Product Management teams regarding issues and suggestions so they can be implemented in future releases.
Regards
Rishabh