afaict cmd/sc_opentemplate.png is missing for the Colibre icon theme, but is found for the other themes
Rizal Muttaqin committed a patch related to this issue.
It has been pushed to "master":
Colibre: tdf#146761 add Edit Template icons
It will be available in 7.4.0.
The patch should be included in the daily builds available at
https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.