DOC: Document progressbar_theme argument in pm.sample

pymc/sampling/mcmc.py

@@ -546,6 +546,23 @@ def sample(
                 are also displayed.
 
             If True, the default is "split+stats" is used.
+
+    progressbar_theme : rich.theme.Theme, optional
+        Custom theme applied to the Rich Console used for PyMC progress bars.
+
+        Colors and styles must follow Rich's style syntax. Not all common color
+        names are supported and invalid values may raise a ``StyleSyntaxError``.
+
+        For reference, see the Rich documentation:
+        - `Rich style syntax <https://rich.readthedocs.io/en/stable/style.html>`_
+        - `Supported color names <https://rich.readthedocs.io/en/stable/appendix/colors.html>`_
+
+        Note
+        ----
+        The progress bar fill color itself is controlled internally by PyMC and
+        cannot be customized via the theme. The theme affects console styles such
+        as table headers, elapsed/remaining time.
+
     step : function or iterable of functions
         A step function or collection of functions. If there are variables without step methods,
         step methods for those variables will be assigned automatically. By default the NUTS step