Invalid `theme` causes Rendering failed, but an invalid `default-theme` does not

[hollow-owl]

Problem

Invalid theme causes an error and building the book fails, however an invalid default-theme does not. I would expect that an invalid default-theme would also fail to build.

Steps

1. In book.toml add

[output.html]
theme = "does-not-exist"

2. Try to run mdbook serve
3. As expected it gives an error

$ mdbook serve
 INFO Book building has started
 INFO Running the html backend
ERROR Rendering failed
        Caused by: theme dir /scratch/does-not-exist does not exist

5. Now, in book.toml change theme to default-theme

[output.html]
default-theme = "does-not-exist"

5. Run mdbook serve
6. It succeeds but the css is broken with links looking the same as normal text and a weird header

$ mdbook serve
 INFO Book building has started
 INFO Running the html backend
 INFO HTML book written to `/scratch/book`
 INFO Serving on: http://localhost:3000
 INFO Watching for changes...

Possible Solution(s)

Make default-theme fail to build the book like a normal theme does

Notes

No response

Version

mdbook v0.5.3

[hollow-owl]

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