Bird UI (#41)

* Update en.yml

Ultra accessible

* Update layout-single-column.scss

* Update layout-multiple-columns.scss

* Add BirdUI accessible

* Update themes.yml

config/themes.yml

@@ -4,3 +4,4 @@ mastodon-light: styles/mastodon-light.scss
 mastodon-bird-ui: styles/mastodon-bird-ui-dark.scss
 mastodon-bird-ui-light: styles/mastodon-bird-ui-light.scss
 mastodon-bird-ui-contrast: styles/mastodon-bird-ui-contrast.scss
+mastodon-bird-ui-a11y: styles/mastodon-bird-ui-accessible.scss