Fix handling of single font name in theme (#7967)

* Fix handling of single font name in theme

Previous check failed because `str` is an instance of Iterable so was never correctly wrapped in `[...]`. Instead, providing a single font such as "Arial" would result in a font list of "A, r, i, a, l" in the bundled CSS.

* Handle single fonts.Font instance correctly

* add changeset

* changes

---------

Co-authored-by: gradio-pr-bot <gradio-pr-bot@users.noreply.github.com>
Co-authored-by: Ali Abid <aliabid94@gmail.com>

.changeset/yummy-rocks-relax.md

@@ -0,0 +1,5 @@
+---
+"gradio": minor
+---
+
+feat:Fix handling of single font name in theme

gradio/themes/base.py

@@ -462,13 +462,13 @@ def expand_shortcut(shortcut, mode="color", prefix=None):
         self.text_xxl = text_size.xxl
 
         # Font
-        if not isinstance(font, Iterable):
+        if isinstance(font, (fonts.Font, str)):
             font = [font]
         self._font = [
             fontfam if isinstance(fontfam, fonts.Font) else fonts.Font(fontfam)
             for fontfam in font
         ]
-        if not isinstance(font_mono, Iterable):
+        if isinstance(font, fonts.Font) or isinstance(font_mono, str):
             font_mono = [font_mono]
         self._font_mono = [
             fontfam if isinstance(fontfam, fonts.Font) else fonts.Font(fontfam)