/* card bg colour for selected messages */
tr[is="thread-card"].selected .card-container {
     color: black !important;
     background-color: lightgreen !important; 
}

/* row bg colour for selected messages table view*/
tr[is="thread-row"].selected {
  color: white !important;
  background-color: red !important; 
}

This can be done with css, for cards or table view (see picture): <pre>/* card bg colour for selected messages */ tr[is="thread-card"].selected .card-container { color: black !important; background-color: lightgreen !important; } /* row bg colour for selected messages table view*/ tr[is="thread-row"].selected { color: white !important; background-color: red !important; }</pre> Help/Troubleshooting Info, Profile Folder, Open Folder, close TB, create a new folder named chrome, create a new document in chrome with a text editor, name it userChrome.css, Save as type: All files *.*, copy in the above code, change the colours as desired. Double-click ''toolkit.legacyUserProfileCustomizations.stylesheets'' to true in Settings/General/Config. editor, restart TB. [https://www.userchrome.org/download-userchrome-css.html Online css generator] [https://www.userchrome.org/how-create-userchrome-css.html video on how to create a css file (Firefox and TB)]

Thanks that worked a charm.

Further card customization is provided in these topics: https://support.mozilla.org/en-US/questions/1447610 https://support.mozilla.org/en-US/questions/1455376