Change default theme to light gray

What does this MR do and why?

Sets the default theme to Light Grey. Theme can still be configure per instance via config, and per user via preferences.

Why

• The default theme is used for a logged-out user and this neutral theme works best there.
• Also the new navigation was designed witht the light grey theme as first/default theme.

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Log out of your GDK and visit a public page, like http://gdk.test:3000/root
• Also verifiy that you can still set a custom theme in your user preferences.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.