Is it possible, with DocView or pdf-tools or something else, to recolour pdfs (e.g. by inverting or choosing a particular colour for background and foreground)? It's a bit annoying when using a dark colour theme to have to view pdfs with white backgrounds.
Yes, there's a dark ("midnight") theme with
Right-click somewhere within the displayed pdf, go to "Render Options" in the menu (about half-way down) and select "Midnight mode".
M-x pdf-view-midnight-minor-mode RET.
The colours used by the "midnight mode" are specified by the variable
pdf-view-midnight-colors (see the docstring with
C-h v pdf-view-midnight-colors RET — in brief the format of the variable is
(FOREGROUND . BACKGROUND)).
You may also need to adjust the colour of the margins, mode-line and other elements of Emacs's UI.