Kolourpaint uses the icons from the icon theme.

(In reply to David Redondo from comment #1)
> Kolourpaint uses the icons from the icon theme.

Did you mean icon theme from Qt5 Settings? That only affects top row toolbar icons (New, Open, Save, etc). I was talking about the toolbar icons at the left panel (selection, text, line, flood fill, etc).

Oh right, I think these are icons shipped by kolourpaint that are used if the theme does not include these icons

Yeah it falls back to Oxygen icons, which do indeed not support this very well.

Also happens on Linux Mint with MATE DE

On Linux Mint MATE when I change to any of the default dark themes:
"Mint-Y-Dark",
"Mint-Y-Dark-Red",
"Mint-Y-Dark-Aqua",
"Mint-Y-Dark-Blue",
"Mint-Y-Dark-Brown",
"Mint-Y-Dark-Grey",
"Mint-Y-Dark-Orange",
"Mint-Y-Dark-Pink",
"Mint-Y-Dark-Purple",
"Mint-Y-Dark-Sand",
"Mint-Y-Dark-Teal"

(In reply to David Redondo from comment #3)
> Oh right, I think these are icons shipped by kolourpaint that are used if
> the theme does not include these icons

If you want, I can edit the icons (should be easy, just changing the icon color to light gray or white).

The bug is not distro-specific. I can confirm this bug on recent KDE Neon with dark theme (Breeze Dark) and Oxygen icons.

Operating System: KDE neon 5.23
KDE Plasma Version: 5.23.4
KDE Frameworks Version: 5.89.0
Qt Version: 5.15.3
Kernel Version: 5.11.0-41-generic (64-bit)
Graphics Platform: X11

Also happens on Pop OS 22.04 
Version 22.12.1 (flatpak version)

Created attachment 159484 [details]
KolourPaint with Oxygen and dark color scheme

This is a problem, but not in the way it's described here. Most of the icons seen in the attachment screenshot are the bundled fallback icons KolourPaint has in case the active icon theme lacks the necessary icons. These icons are from KDE3 and predate Oxygen. The rest of the icons seem to be from Breeze (not Breeze Dark), which might be because the icon theme being used was missing KolourPaint's Tool Box icons and falling back to Breeze as a result. When Breeze icons are used as fallback, the light version is always used regardless of the color scheme, so on dark color schemes you get icons that are very difficult to see. I'm not sure why only some icons in the attachment are falling back to Breeze, while others are falling back to the ones packaged with KolourPaint. Both Breeze and KolourPaint have all of the necessary icons, so there should only be one level of fallback. Still, the important part is that none of the icons from the attached screenshot are from Oxygen, so the bug the person who opened this was facing probably had nothing to do with Oxygen.

Despite that, this is still an issue Oxygen has. Oxygen is missing several of KolourPaint's Tool Box icons. To be specific, it's missing tool_freeform_selection, tool_elliptical_selection, tool_line, tool_color_eraser, tool_spraycan, tool_rounded_rectangle, tool_polygon and tool_curve. For all of those icons, it ends up falling back to Breeze, so those icons end up being hard to see, because again, Breeze fallbacks for icons are always Breeze light, even on a dark color scheme. I've attached a screenshot of KolourPaint with Oxygen and a dark color scheme to show the difference.

(In reply to poudinkpopinski from comment #10)
> Created attachment 159484 [details]
> KolourPaint with Oxygen and dark color scheme
> 
> This is a problem, but not in the way it's described here. Most of the icons
> seen in the attachment screenshot are the bundled fallback icons KolourPaint
> has in case the active icon theme lacks the necessary icons. These icons are
> from KDE3 and predate Oxygen. The rest of the icons seem to be from Breeze
> (not Breeze Dark), which might be because the icon theme being used was
> missing KolourPaint's Tool Box icons and falling back to Breeze as a result.
> When Breeze icons are used as fallback, the light version is always used
> regardless of the color scheme, so on dark color schemes you get icons that
> are very difficult to see. I'm not sure why only some icons in the
> attachment are falling back to Breeze, while others are falling back to the
> ones packaged with KolourPaint. Both Breeze and KolourPaint have all of the
> necessary icons, so there should only be one level of fallback. Still, the
> important part is that none of the icons from the attached screenshot are
> from Oxygen, so the bug the person who opened this was facing probably had
> nothing to do with Oxygen.
> 
> Despite that, this is still an issue Oxygen has. Oxygen is missing several
> of KolourPaint's Tool Box icons. To be specific, it's missing
> tool_freeform_selection, tool_elliptical_selection, tool_line,
> tool_color_eraser, tool_spraycan, tool_rounded_rectangle, tool_polygon and
> tool_curve. For all of those icons, it ends up falling back to Breeze, so
> those icons end up being hard to see, because again, Breeze fallbacks for
> icons are always Breeze light, even on a dark color scheme. I've attached a
> screenshot of KolourPaint with Oxygen and a dark color scheme to show the
> difference.

maybe the falback should go to the correct variant. even if oxygen is missing those icons a better fix would be to make sure the fall back is sane

Created attachment 177897 [details]
multiple missing icons in Kolourpaint

I face a similar problem with Kolourpaint when combined with the Oxygen icon set. Some icons don't appear at all. Please see the latest attached screenshot.

This was tested in:
Fedora 41 KDE
Plasma: 6.2.5
Frameworks: 6.10.0
Qt: 6.8.1
Wayland

I'm guessing some one must be messing with the name conventions and changing the name of the icons ... Or the bundle you are using of oxygen icons has many removed..

It could be a Fedora specific problem as I just fired up a VM with Solus KDE and all the Oxygen icons seems to work fine there.

I am using the official Oxygen icon theme btw:
https://github.com/KDE/oxygen-icons/releases/tag/v6.1.0

Dunno, but lately a lot of issues have been popping out that are in this vain... Please let me know what you find out...

A lot of distros don't want to ship oxygen as it's a rather large ste specially if the source files are included...

(In reply to pinheiro from comment #16)
> Dunno, but lately a lot of issues have been popping out that are in this
> vain... Please let me know what you find out...
> 
> A lot of distros don't want to ship oxygen as it's a rather large ste
> specially if the source files are included...

Just for the record I am not having this problem with Kolourpaint anymore. It was resolved with a fresh download of the Oxygen icons tar.gz, as was the other similar bug I was facing with some application icons:
https://bugs.kde.org/show_bug.cgi?id=499408