[3.12] gh-111050: IDLE - Simplify configdialog.HighPage.theme_elements (GH-111053) (#111055)

gh-111050: IDLE - Simplify configdialog.HighPage.theme_elements (GH-111053)

Replace tuple value with internal name, removing numbers.
Remove sorting of already ordered dislay names.
Remove '[0]' indexing into now-gone tuple.
(cherry picked from commit 642eb8df951f2f1d4bf4d93ee568707c5bf40a96)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>