Fix various theming issues on bright colors

[juliusknorr]

Theming fixes from #14639

• Light theming color:
  • On error pages like "File not found"
  • "Forgot password?" on log in is light, should be dark
  • App name in header has boxy background
  • 3 dot icon in share link header is white, should be black
  • First run wizard "next" icon should be dark because the primary color is light -> Fix next arrow on light theming colors firstrunwizard#182
• Dark theme: "File not found" screen has dark icon, should be white
• Basically all the content of the .body-login-container should always be white because the container background is dark (#body-login p rule for footer overwrites this, should be more specific)

[ChristophWurst]

Fine by me otherwise

(gave it a quick test and it looked good)

[skjnldsv]

Looks good!