If you want to dive into the widget tree and fiddle with fonts and colors and such, the class descriptions are in the table below. I also highly recommend the tkinspect program, which you can find in the Tcl archives.
Main Top row of buttons and title label
Fdisp Folder label display
Fltop Folder label display when it
is in a detached top level
Fops Folder operation buttons and folder label
Ftoc Folder table of contents display
Msg Message display
Clip Detached message display
Sedit Simple editor top-levels
Help Help window
Key Color key window
Pref Preferences dialogs
Log Error/debug log
Pick The pick dialog
NewFolder The new folder dialog
DeleteFolder The delete folder dialog
WhatNow The What Now? dialog
Error Error popups
Dialog General popups
Mid Frame around Face, Msgid, Status, Mops
Mid.Face Bitmap display
Mid.Right.Status.label Message label
Mid.Right.Status.msg Status line
Mid.Right.Mops Message buttons