I know this thread was more specifically about a dark theme for the entire HubSpot app. But since this was posted in the CMS Development forum, I just wanted to add that there are themes available on the settings page in the Design Manager. You can find the settings in the bottom-right of the Design Manager window. If you click on Settings, you can then select the Settings option, and it will bring you to a page where you can select Code from the dropdown. It should look like this:
Under the appearance tab, you’ll have an option to change the theme for code editing.
As for creating a dark theme for the rest of the HubSpot app, you could create an idea for it here: https://community.hubspot.com/t5/custom/page/page-id/ideaslandingpage.