any use of brand causes three stylesheets to be present in output

[gordonwoodhull]

Minimal example

• m-c.qmd

---
title: "Reproducible Quarto Document"
brand:
  typography:
    base:
      size: 14pt
---

This is minimal

This emits light,dark,light <link rel="stylesheet"> or do I see light,light,light?

<link href="m-c_files/libs/bootstrap/bootstrap-1a8a0f34608aab50ebe6d5e40c8a664b.min.css" rel="stylesheet" append-hash="true" class="quarto-color-scheme" id="quarto-bootstrap" data-mode="light">
<link href="m-c_files/libs/bootstrap/bootstrap-dark-1a8a0f34608aab50ebe6d5e40c8a664b.min.css" rel="stylesheet" append-hash="true" class="quarto-color-scheme quarto-color-alternate" id="quarto-bootstrap" data-mode="light">
<link href="m-c_files/libs/bootstrap/bootstrap-1a8a0f34608aab50ebe6d5e40c8a664b.min.css" rel="stylesheet" append-hash="true" class="quarto-color-scheme-extra" id="quarto-bootstrap" data-mode="light">

🤦

This should be harmless but it's wrong and very much like the harmless unshown dark logos in #13020. I think we have inconsistent heuristics for whether dark mode is enabled, and brand now provides dark variants even when dark mode is not enabled through the brand, so dark mode is not enabled but the stylesheets are.

I won't be able to investigate this properly for Monday's patch release, but it is bug introduced in 1.8 and therefore a regression.

I'll need to add proper tests that verify extra stylesheets don't get added.