The wrong/inconsistent coloring of the icons has been fixed.

With the above patch for breeze the toolbar and the configuration dialog title now also look correct in high-contrast mode.

on gpg4win-5.0.0-beta190@win10 (high contrast):

• contrast black

• contrast nr. 2

• normal

Tested: gpg4win-5.0.0-beta330 @ win10

Issues found

• find action (menu: help)

white	black	nr1 (at least not optimal)

• calendar (e.g. selection of cert expiration date)

white	black	nr2

• new smime cert generation dialog

white	black	nr1	nr2

Unsure

• invalid state in cert list tooltip (red on black)

black, nr1, nr2

• smime icon in kleopatra config (was already marked in the description screenshot)

black, nr1, nr2

• green invalid state icon, e.g. on certificate selection (probably as intended?)

nr2

• unselected radio/checkboxes are a bit hard to see and worse to distinguish, a bit more contrast would be better in my opinion

black, nr1, nr2

• the link color on black background feels also a bit hard to read, but it's the same in native windows settings dialogs, so probably to be expected

black, nr1, nr2

Regarding the "unsure" findings:

• invalid state in cert list tooltip (red on black)
  
  That's not okay, i.e. it should be changed/fixed.

• smime icon in kleopatra config (was already marked in the description screenshot)
  
  That's mostly okay. In general, colored icons are acceptable in high contrast. Older Windows apps, e.g. Windows Explorer, have only colored icons and that's apparently fine. I'd still change the S/MIME Validation icon because it looks out-of-place as only colored icon and because I have no idea what the globe has to do with S/MIME Validation. There has to be a better symbolic icon for this.

• green invalid state icon, e.g. on certificate selection (probably as intended?)
  
  That's okay, i.e. it's as intended. The icons are colored as normal text. Green is just a very bad choice as normal text color. The alternative would be using icons with hard-coded colors. I need to discuss this with the KDE Visual Design Group.

• unselected radio/checkboxes are a bit hard to see and worse to distinguish, a bit more contrast would be better in my opinion
  
  That's not okay, i.e. this needs to be improved. This is one of the reasons why I think that the KDE Breeze style is just not suitable for high-contrast. On the other hand, KDE is looking into such accessibility problems at the moment.

• the link color on black background feels also a bit hard to read, but it's the same in native windows settings dialogs, so probably to be expected
  
  That's okay, i.e. it's as intended. This should be the color that the Windows high contrast color scheme defines as color for links.

Tested: gpg4win-5.0.0-beta330 @ win11

Thanks for the feedback. The issues are the same as on win10:

• find action

abendämmerung	nachthimmel	wasser	wüste

• calendar (18th is mouse hover, 25th is selected)

abendämmerung	nachthimmel	wasser	wüste

• new smime cert generation dialog (same)
• invalid state in cert list tooltip (same)
• smime icon in kleopatra config (same)

In general win11 contrast modes look better and don't have those issues:

• green/yellow invalid state icon (all icons are black/white now; note the issue with missing borders in nachthimmel signature validation)

abendämmerung	nachthimmel	wasser	wüste

• unselected radio/checkboxes contrast is better (exception: nachthimmel)

abendämmerung	nachthimmel	wasser	wüste

Addionally

• the "folder up" icon of the open file dialog looks disabled in all dark modes (on wüste it's ok)

Thanks for the Windows 11 screen shots.

Re: issue with missing borders in nachthimmel signature validation
-> This is also present in all dark Windows 10 contrast modes. The border has the same color as the background (pitch black aka #000000). The technical reason is that anything times zero yields zero. That's easy to fix by special-casing the case #000000.

Re: the "folder up" icon of the open file dialog looks disabled in all dark modes
-> This is the standard Windows file dialog (which is basically the Windows Explorer without the menu bar). There's nothing we can do.