diff --git a/CHANGELOG.md b/CHANGELOG.md
index ee99450f9..446f4182b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,7 +11,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * Fixed a bug with `Chat()` sometimes silently dropping errors. (#1672)
 
-* `shiny create` now uses the template `id` rather than the directory name as the default directory. (#1666)
+* `shiny create` now uses the template `id` rather than the directory name as the default directory.
+(#1666)
+
+* `ui.Theme()` now works correctly on Windows when the theme requires Sass compilation. (thanks @yuuuxt, #1684)
 
 ## [1.1.0] - 2024-09-03
 
diff --git a/shiny/ui/_theme.py b/shiny/ui/_theme.py
index 5c4ca225b..27c7e772f 100644
--- a/shiny/ui/_theme.py
+++ b/shiny/ui/_theme.py
@@ -510,7 +510,10 @@ def path_pkg_preset(preset: ShinyThemePreset, *args: str) -> str:
     #> "{shiny}/www/shared/sass/preset/shiny/bootstrap.min.css"
     ```
     """
-    return os.path.realpath(path_pkg_www("sass", "preset", str(preset), *args))
+    path = os.path.realpath(path_pkg_www("sass", "preset", str(preset), *args))
+    # these paths end up in strings passed to sass.compile(). Converting to POSIX means
+    # we use forward slashes to avoid needing to double-escape the Windows backslash
+    return pathlib.Path(path).as_posix()
 
 
 def check_is_valid_preset(preset: ShinyThemePreset) -> None: