`collapse-all` icons have no `id="current-color-scheme"` attribute in the `<style>` element, so they do not respect the foreground color and these icons are hardly visible in dark color themes.
Special thanks: David Hurka, who reported the bug and suggested the change
BUG: 409546
Usage in Okular, Before:
{F6848602}
After:
{F6980718}