Add the ability to use custom css

locale/translations.go

locale/translations/de_DE.json

locale/translations/en_US.json

locale/translations/es_ES.json

locale/translations/fr_FR.json

locale/translations/it_IT.json

locale/translations/ja_JP.json

locale/translations/nl_NL.json

locale/translations/pl_PL.json

locale/translations/ru_RU.json

locale/translations/zh_CN.json

storage/user.go

template/common.go

template/html/common/layout.html

template/html/settings.html

template/views.go

ui/form/settings.go

ui/settings_show.go

ui/static_stylesheet.go