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.

Locate the slide-deck files in the repository and read the Appendix in the training manual before changing anything. Done means the slides have a consistently improved visual style, but the issue does not define specific acceptance criteria or name tests.

索引モデルが issue の本文から書いたものです。