The new dark mode introduced with Mojave has some issues with Mails where the author has used text highlighting (like, putting a yellow background behind a black text): This is going to be white on yellow in dark mode, and hence hardly readable. Worse even, there appears to be a trend where some people send mails with a black background in the first place, making them harder to read. Here is a way to quickly change that on a per-mail basis.
Basically, if you chose “Use dark backgrounds for messages” within the settings of Mail, you get an additional menu item in View – Message – Show with Light Background. Would it not be nice to have an option to trigger this menu item with a keyboard combination.
Here is a script that you can run using Better Touch Tool – I put it on Cmd+B (for background) for Mail. It switches back and forth between dark and light background. You’ll probably need to change the labels for your language: There’s a commented line that shows how to do it… For English, it’s probably “Message” instead of “E-Mail”, and “View” instead of “Darstellung.”
tell application "Mail" to activate
tell application "System Events"
tell process "Mail"
click menu item 1 ¬
of menu "E-Mail" of menu item "E-Mail" of menu "Darstellung" of menu bar 1
-- set xxx to name of every menu item of menu "Darstellung" of menu bar 1