Add id="current-color-scheme" to collapse-all icons

Summary:
`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}

Reviewers: #vdg, davidhurka, ndavis

Reviewed By: #vdg, davidhurka, ndavis

Subscribers: ndavis, davidhurka, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D22297