How to specify block placement with theme

If I want to change the placement of a block in a layout, specific to a contrib (or core) theme. How would I do that?

For example, if I hypothetically wanted to create a PR that put the breadcrumb block above the title block in the default layout for the Basis theme, how would I accomplish that?

(This relates to https://github.com/backdrop/backdrop-issues/issues/4113)