I want to provide a listbox where the user can select (multiple) characters (usually close to 15). Quite often some of these will be sequenced, so a listbox is easier than a validated text entry field.
Since the character combination has a meaning to the user, it is user friendly to orient the listbox horizontally.
Is there an easy way e.g. a theme, setting or subclass of the Tkinter listbox so I do not have to build my own?
No, there is no setting, subclass or theme that will let you do that with a listbox.
What you might want to use instead is a set of check buttons with the indicator turned off so they each appear as a button with a single letter. You can then pack them all horizontally in a frame. With the indicator off, the button will appear sunken when selected, or normal otherwise.
Horizontally oriented listbox using Python and Tkinter