30-second answer

PowerShell
$web = Get-PnPWeb -Includes AlternateCssUrl
if ([string]::IsNullOrWhiteSpace($web.AlternateCssUrl)) { "None" }
else { $web.AlternateCssUrl }

What this proves

Whether the site is configured to inject a custom stylesheet into its classic pages, and which file it is.

What it does not prove

  • What the stylesheet changes. The property stores a path, not the intent behind it; the file may recolour a banner or restructure the whole chrome. Reading the CSS itself is a separate step.
  • That modern pages are affected. They are not: alternate CSS applies to the classic experience. Its presence tells you about the site's past, and about what classic pages still look like.

PowerShell

PowerShell
Get-PnPWeb -Includes AlternateCssUrl, MasterUrl |
  Select-Object @{n='AlternateCss'; e={ $_.AlternateCssUrl }},
                @{n='MasterFile';  e={ Split-Path -Leaf $_.MasterUrl }}

Read it together with the master page: the two are the classic branding pair, and a site with either has been deliberately styled.

Example output

Text
AlternateCss                              MasterFile
------------                              ----------
/sites/intranet/Style Library/corp.css    seattle.master

Explanation

An empty value is the answer on most sites, and empty means observed-empty: the property loaded and there is no stylesheet. A populated value means somebody invested in classic branding, and that investment is the part of a migration that does not carry over. Modern sites express branding through themes and site designs; a corp.css does not translate, it gets rebuilt or retired.

Production considerations

  • Load with -Includes; an unloaded property returns null and is indistinguishable from "no stylesheet" unless you asked for it.
  • The path may point at the Style Library of the site or of another site. A stylesheet hosted centrally means the branding, and the blast radius of changing it, is shared.
  • SPO-MODERN-004: the site loads an alternate stylesheet into every page

References

Found something wrong? Suggest a correction. The article source is not public; the engine it cites is.

What this answer underwrites

The engine reads these Microsoft operations to collect evidence, so what is established here is what those collectors rest on.

  • activitywhen a person last changed something on one site
  • agentsthe Copilot agents in one site, and the sources each declares
  • classificationwhat a site records about the kind of content it holds
  • modernityhow one site is built: template, branding, publishing
  • ownerswho administers one site
  • sharingwhat one site permits, and its default link