Resource detection is broken for custom format using an overwrite of theme argument

[cderv]

From r-lib/pkgdown#2319 cc @gadenbuie

---
output: 
  cleanrmd::html_document_clean: 
    theme: no-class
---

# test

And you'll have an issue

> rmarkdown::find_external_resources("test.Rmd")
Error in `match.arg()`:
! 'arg' should be one of “default”, “bootstrap”, “cerulean”, “cosmo”, “darkly”, “flatly”, “journal”, “lumen”, “paper”, “readable”, “sandstone”, “simplex”, “spacelab”, “united”, “yeti”
Run `rlang::last_trace()` to see where the error occurred.
> rlang::last_trace()
<error/rlang_error>
Error in `match.arg()`:
! 'arg' should be one of “default”, “bootstrap”, “cerulean”, “cosmo”, “darkly”, “flatly”, “journal”, “lumen”, “paper”, “readable”, “sandstone”, “simplex”, “spacelab”, “united”, “yeti”
---
Backtrace:
    ▆
 1. └─rmarkdown::find_external_resources("test.Rmd")
 2.   └─rmarkdown:::discover_rmd_resources(input_file, discover_single_resource)
 3.     ├─rmarkdown:::is_bs_theme(resolve_theme(output_format[["theme"]]))
 4.     │ └─bslib::is_bs_theme(theme)
 5.     └─rmarkdown:::resolve_theme(output_format[["theme"]])
 6.       └─base::match.arg(theme, themes())

This is because

• find_external_resources() parsed the YAML front matter to do some resource detection
• in detecting external resources needs to consider css argument #2486 we added a check on theme for bslib resource handling, and we used resolve_theme()
• resolve_theme() was made for handling bslib compatible theme argument and not custom ones.

We should probably use a tryCatch() in discover_rmd_resources() because if resolve_theme() errors, this means this is not a bslib theme for sure.

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