Rollup merge of #83705 - jyn514:theme-error, r=GuillaumeGomez

Give a better error when --theme is not a CSS file

Before:

```
error: invalid argument: "bacon.toml"
```

After:
```
error: invalid argument: "bacon.toml"
  |
  = help: arguments to --theme must be CSS files
```

cc https://github.com/rust-lang/rust/pull/83478

diff --cc src/librustdoc/config.rs
Simple merge

diff --cc src/tools/compiletest/src/runtest.rs
Simple merge