Restyle the Slides

## Do you have an eye for design?
Help us improve the look of the slide deck.
If you need help, check out the Appendix in the training manual.

Review the slide deck and the Appendix in the training manual before deciding what visual changes are needed. The work is done when the deck has a consistently improved look, but the issue does not name files, slides, or concrete design criteria.

索引モデルが issue の本文から書いたものです。