If this is due to upgrading to 115, some CSS customizations no longer work.

If by 'mailbox window' you mean the Threads Pane, the background can be changed with userChrome.css:

table[is="tree-view-table"]{
background: #87CEFA !important;}

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 (userContent.css), Save as type: All files *.*, copy in the above code, change the colour as desired. Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Settings/General/Config. editor, restart TB.

https://www.userchrome.org/download-userchrome-css.html

Wow! I had quite a time figuring out your extremely terse instructions and googling various terms, but I managed to work my way through it and it worked!

My only quibble (apart from the extreme economy of your instructions is with "userChrome.css (userContent.css)". I couldn't figure out what the parenthetical userContent.css was all about - so I just went with userChrome.css and it worked.

Thank you.

Wow! I had quite a time figuring out your extremely terse instructions and googling various terms, but I managed to work my way through it and it worked! My only quibble (apart from the extreme economy of your instructions is with "userChrome.css (userContent.css)". I couldn't figure out what the parenthetical userContent.css was all about - so I just went with userChrome.css and it worked. Thank you.

The instructions apply to both userChrome and userContent files. An example of the latter changes the scrollbar colours and width in the Message Pane and Write windows:

:root { 
    scrollbar-color: grey #87CEFA;
    scrollbar-width: 25px;}

To change the background and/or text colours of the Folder Pane to match the Threads Pane, add this to userChrome:

#folderTree
{background-color: #87CEFA !important;
  color: navy !important;}