Allow custom CSS for Snipvault UI

Please allow custom CSS so Snipvault can work with all admin themes and UI preferences. I can’t use SV because the WP sidebar menu covers the SV UI, and I absolutely need the sidebar and the top menu bar to be present when using SV.

Because it runs in an iFrame we can’t use a theme file or external plugin to override the SV styling, so the only I am able to make it work is to modify the plugin to add custom styles to the styles.css file and re-install the plugin.