Optimise + light/dark mode

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

Author: hugovk

peps/pep-0012.rst

@@ -601,6 +601,8 @@ processed output using the ``image`` directive:
 
 Any browser-friendly graphics format is possible; SVG or PNG are
 preferred for graphics, JPEG for photos and GIF for animations.
+Images should be optimised to reduce their file size, and should
+be legible in both light and dark mode in the browser.
 
 For accessibility and readers of the source text, you should include
 a description of the image and any key information contained within