CSS tweaks to typography and layout of post page

src/app/assets/stylesheets/_themes.scss

src/app/components/App.scss

src/app/components/cards/PostFull.jsx

src/app/components/cards/PostFull.scss

src/app/components/elements/DropdownMenu.scss

src/app/components/elements/ReplyEditor.jsx

src/app/components/elements/ReplyEditor.scss

src/app/components/elements/TagList.scss

src/app/components/pages/Post.scss

src/app/components/pages/PostsIndex.scss

src/app/components/pages/Welcome.jsx

src/app/components/pages/Witnesses.scss