Define default fonts for Gradio theme

Allows web UI to (almost) be ran fully offline.
The web UI will hang on load if offline when
these fonts are not manually defined, as it will attempt (and fail)
to pull from Google Fonts.

modules/shared.py

@@ -666,13 +666,19 @@ def reload_gradio_theme(theme_name=None):
         theme_name = opts.gradio_theme
 
     if theme_name == "Default":
-        gradio_theme = gr.themes.Default()
+        gradio_theme = gr.themes.Default(
+            font=['Helvetica', 'ui-sans-serif', 'system-ui', 'sans-serif'],
+            font_mono=['IBM Plex Mono', 'ui-monospace', 'Consolas', 'monospace'],
+        )
     else:
         try:
             gradio_theme = gr.themes.ThemeClass.from_hub(theme_name)
         except Exception as e:
             errors.display(e, "changing gradio theme")
-            gradio_theme = gr.themes.Default()
+            gradio_theme = gr.themes.Default(
+                font=['Helvetica', 'ui-sans-serif', 'system-ui', 'sans-serif'],
+                font_mono=['IBM Plex Mono', 'ui-monospace', 'Consolas', 'monospace'],
+            )