My apologies. On my first read through I must have misread. I thought that you were trying to make it look like a Ulysees theme
The theme for the above screenshot looks like it is one called Vivid Dark, though honestly, that screenshot does not look as vivid to me as whern the theme is selected on my iPad.
Themes are a pro subscription feature and as @Andreas_Haberle pointed out, you access the settings for that from the ‘Aa’ icon at the bottom of the Drafts screen.
Vivid Dark, as you might imagine is one of Drafts’ “dark” application themes. If you want to display in this theme all the time, following the instructions in the Drafts documentation linked below, set your Active Theme to Dark (it is just more logical than specifying it as a light theme), and set your Theme Selection to the Dark theme Vivid Dark.