Super simple snippet that replaces white background in a pdf with a grey one using pymupdf

grey_pdf_bg.py

# To make those white background PDFs a little easier on the eyes
import pymupdf

doc = pymupdf.open('path/to/pdf-in.pdf')
background_color = (211/255, 211/255, 211/255)

for page in doc:
    page.draw_rect(page.rect, color=None, fill=background_color, overlay=False)

doc.ez_save('path/to/pdf-out.pdf')
doc.close()

# The below is a crude attempt at a true dark theme without affecting images. 
# It doesn't work very well, but maybe could with some effort.
# There are some solutions to dark-theme for PDFs, 
# but they tend to invert colors for everything instead of just text. 
# I've also seen file sizes balloon.

# import pymupdf
# from pymupdf import Font
# from collections import defaultdict


# doc = pymupdf.open('path/to/pdf-in.pdf')

# background_color = (0/255, 0/255, 0/255) # black
# text_color = (255/255, 255/255, 255/255) # white

# fonts = defaultdict(lambda: 0)
# default_font = Font('helv')

# for page in doc:
#     page.draw_rect(page.rect, color=None, fill=background_color, overlay=False)
    
#     spans = []
#     for block in page.get_text("dict")['blocks']:
#         for line in block.get('lines', []):
#             for span in line['spans']:
#                 if span.get('color', 0) != 0:
#                     continue
#                 spans.append(span)
#                 page.add_redact_annot(span['bbox'])
                
#     page.apply_redactions()
    
#     tw = pymupdf.TextWriter(page.rect, color=text_color)
    
#     for span in spans:
#         font = span['font']
#         fonts[font] += 1
#         tw.append(span["origin"], span["text"], font=default_font, fontsize=span["size"])
    
#     tw.write_text(page)
                

# doc.ez_save('path/to/pdf-out.pdf')
# doc.close()