Make `css` configuration behave more like `theme` to avoid confusion due to bad path handling

I have:

• searched the issue tracker for similar issues
• installed the latest version of Quarto CLI
• formatted my issue following the Bug Reports guide

Bug description

Quarto will find the file regardless of the path being wrong (what matters is the name of the file).

Steps to reproduce

echo -e '---\ntitle: "Hello, World!"\nformat: html\ncss: ../../../../../../file.css\n---\n' > index.qmd
echo 'h1 { color: red; }' > file.css
quarto render index.qmd

Actual behavior

The CSS file is somehow found and used.

Expected behavior

A warning should be raised and the document should not get the styling from the file.

Your environment

• IDE: VSCode latest
• Ubuntu latest and macOS latest
• Quarto 1.8 and development version

Quarto check output

Quarto 99.9.9
[✓] Checking Quarto installation......OK
      Version: 99.9.9
      commit: 3b905a23928ddcd582edcc9ace978a9294f15558
      Path: /Users/mcanouil/Projects/quarto-dev/quarto-cli/package/dist/bin

[✓] Checking tools....................OK
      TinyTeX: v2025.11
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /Users/mcanouil/Library/TinyTeX/bin/universal-darwin
      Version: 2025

[✓] Checking Chrome Headless....................OK
      Using: Chrome found on system
      Path: /Applications/Brave Browser.app/Contents/MacOS/Brave Browser
      Source: QUARTO_CHROMIUM

[✓] Checking basic markdown render....OK

Start by reproducing the issue with index.qmd, file.css, and the quarto render index.qmd command. Trace how the css configuration resolves the referenced path and compare that behavior with theme. Done means an invalid path produces a warning and the CSS file is not applied.