v2.0.0 — Switchable color themes

Switchable color themes

The color scheme is no longer hardcoded: pick a theme with a single line in
the preamble of slide.tex.

\usepackage{slidestyle}
\slidetheme{agu} % omit for the default (oxford)

Theme	Base	Accent
oxford (default)	Oxford Blue #002147	Cerulean Blue #49B6FF
agu	Aoyama Gakuin dark green #025F3D	blue-green #00A384
cambridge	Dark Teal #133844	Cambridge Blue #8EE8D8
mit	MIT Red #A31F34	Silver Gray #8A8B8C
princeton	Black #000000	Princeton Orange #E77500

All values come from each university's official palette; the Aoyama Gakuin
pair is measured from its official logo assets (no published color codes
exist). Adding a theme takes two \definecolor lines
(<name>Primary / <name>Accent) in §2 of slidestyle.sty.

Breaking changes

• Default look changed from Aoyama Gakuin dark green to Oxford Blue.
  Use \slidetheme{agu} to restore the previous colors (now corrected to
  the official values, so they differ slightly from v1.0.0).
• Color names agu / agu2 removed — decks that referenced them
  should use themeprimary / themeaccent instead.
• Color name kenbridge renamed to cambridge (correct spelling).
• The sample deck's "Review" section is now "Math Environments",
  reflecting its intent as a math-environment showcase.

Layout

• The "# event name" ribbon on the title page is smaller
  (\footnotesize), and the title block is re-centered accordingly.

Documentation

• New "Theme colors" section in the root README, with refreshed
  screenshots showing the Oxford default.
• Per-template READMEs (ja/en) document the switching mechanism.

Prebuilt PDFs (slide-ja.pdf / slide-en.pdf) are attached by CI.