Zoom in to Full Page width, or change Window Size, so PDF reaches edge (with Dark Reader from your question):
For example, on https://www.gao.gov/assets/gao-19-221.pdf (*):
Now you still have the "gray color border" imposed by Google Chrome (whose bright idea was that, was it a full thought? We can blame trees for white paper.), but you're not forced to take that as the full version.
Ideally being a Black background would be obvious, computer code would enable us to disable these unconsciously scripted forms to be self-edited, it's not just a form of accessibility, it's about designing to be accessible in the lowest code logic.