CSS tweaks for Vivaldi Forums

This is a userscript with various CSS tweaks for the forums at: https://forum.vivaldi.net

• Saving vertical space by making menus and headers more compact
• Removing a lot of padding and margins
• Making chats & notifications dropdown longer
• Bigger search field
• Proper accessibility outline for focused input fields
• Fixes for dark theme colour contrast
• Showing warning for external links (moderation purposes)
• Etc...

Input/suggestions appreciated.