You can extend a single column, but not columns in general, I think.
columns:
customcol:
extends: columns/costumcol
Or you can extend a complete layout, no matter if it contains tabs or columns.
You can extend a single column, but not columns in general, I think.
columns:
customcol:
extends: columns/costumcol
Or you can extend a complete layout, no matter if it contains tabs or columns.