#563 - combined light and dark theme style.css + extracted sidenav.css

src/main/resources/static/css/colors.css

src/main/resources/static/css/dark/sidenav.css

src/main/resources/static/css/dark/style.css

src/main/resources/static/css/sidenav.css

src/main/resources/static/css/style.css

src/main/resources/static/css/templates.css

src/main/resources/templates/helpers/header.ftl