Clean up, use translations, persist theme, add more presets.

src/components/settings/settings.js

src/components/style_switcher/style_switcher.js

src/components/style_switcher/style_switcher.vue

src/i18n/messages.js

src/lib/persisted_state.js

src/main.js

src/services/style_setter/style_setter.js

static/styles.json