Allow specifying multiple CSS files in themes (#10465)

Co-authored-by: Adam Turner <9087854+aa-turner@users.noreply.github.com>

tests/roots/test-build-html-theme-having-multiple-stylesheets/_themes/mytheme/theme.conf

@@ -0,0 +1,3 @@
+[theme]
+inherit = basic
+stylesheet = mytheme.css, extra.css

tests/roots/test-build-html-theme-having-multiple-stylesheets/conf.py

@@ -0,0 +1,2 @@
+html_theme_path = ['_themes']
+html_theme = 'mytheme'

tests/roots/test-build-html-theme-having-multiple-stylesheets/index.rst

@@ -0,0 +1,2 @@
+test-build-html-theme-having-multiple-stylesheets
+=================================================

tests/test_build_html.py

@@ -1782,3 +1782,12 @@ def test_theme_options_with_override(app, status, warning):
     result = (app.outdir / '_static' / 'documentation_options.js').read_text(encoding='utf8')
     assert 'NAVIGATION_WITH_KEYS: true' in result
     assert 'ENABLE_SEARCH_SHORTCUTS: false' in result
+
+
+@pytest.mark.sphinx('html', testroot='build-html-theme-having-multiple-stylesheets')
+def test_theme_having_multiple_stylesheets(app):
+    app.build()
+    content = (app.outdir / 'index.html').read_text(encoding='utf-8')
+
+    assert '<link rel="stylesheet" type="text/css" href="_static/mytheme.css" />' in content
+    assert '<link rel="stylesheet" type="text/css" href="_static/extra.css" />' in content