The existing light theme has relatively low contrast between the text
(and other UI elements) and background (especially within code blocks).
This presents difficulties for people with reduced visual contrast
perception (common in older adults).
This patch makes changes to the default `light` theme to meet the
minimum contrast requirement of the v2.1 W3C WCAG (Web Content
Accessibility Guidelines)
https://www.w3.org/WAI/WCAG21/quickref/#contrast-minimum
The small size, and slender font used for the title text makes it hard
to read, even with the increased contrast colour scheme, so this patch
also increases the size of the title text font by 20%.
Closes#1442
