/* The To: email addresses */
#MsgHeadersToolbar .address-pill  {
  background-color: #336699 !important;
  color:            White   !important;
  height:           40px    !important;
  font-size: 18px !important;
}

/* The To: address "input" box (ie with cursor) */
#MsgHeadersToolbar .recipients-container input {
  border-radius:    3px     !important;
  color:            white   !important;
  background-color: #336699 !important;
  padding-inline:   2px     !important;
  font-size: 18px !important;
}

There might be a simpler way to do this, but in the meantime I found some css code that works. <pre> /* The To: email addresses */ #MsgHeadersToolbar .address-pill { background-color: #336699 !important; color: White !important; height: 40px !important; font-size: 18px !important; } /* The To: address "input" box (ie with cursor) */ #MsgHeadersToolbar .recipients-container input { border-radius: 3px !important; color: white !important; background-color: #336699 !important; padding-inline: 2px !important; font-size: 18px !important; }</pre> https://www.elevenforum.com/t/thunderbird-115-css-customization.16320/post-342200 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 numbers and 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)] See the attached pictures that show the effect with and without the css.