You probably need to recreate your grey material icon theme .rcc but update it to use the current set of svg files used by Sigil. Try looking at master in the Sigil IconThemes repo for more info.
I have no idea how Becky added text to those buttons that could also be translated in any way.
Probably should ask BeckyEbook how to best go about recreating your custom theme with the right colour, updated svg icons and how she adds text to those buttons.
Update:
I looked at the Sigil IconThemes repository in its Releases here:
https://github.com/Sigil-Ebook/IconThemes/releases
And the latest release includes both the new webprinter busy/working icon, and the cycle css icon. If you are missing the cycle-css icon you are probably missing the busy-working one which came even later.
And there is a material-gray set that should be complete you can grab from there if that helps.