https://addons.thunderbird.net/en-US/thunderbird/addon/textmarker/

https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

DOING WHAT YOU SAID ONLY CHANGES THE ENTIRE SCREEN BACKGROUND ... NOT JUST THE HIGHLIGHTED TEXT IN THE EMAIL.

THANKS THOUGH ....

It works here (see picture). Note the description for the add-on:

"(Important remark: If the Thunderbird option "Override the colours specified by the content with my selections above:" is set to "Always", the Add-on will not work.)"

Options/General/Language & Appearance, Colors...

Alternative method, without add-ons:

https://support.mozilla.org/en-US/questions/1287456