Show warning when ui.dark_mode breaks Tailwind (#3994)

As discussed in #3753, `ui.dark_mode` can break Tailwind styling if used
on a page with `dark=None`
```py
@ui.page('/', dark=None)
def page():
    ui.dark_mode(True)
    ui.label('This should be bordered').classes('border')
```
or with `ui.run(dark=None)`.

This PR shows a warning in this situation. I won't link it with the
issue because this isn't a proper fix.

Author: falkoschindler

nicegui/elements/dark_mode.py

@@ -1,5 +1,6 @@
 from typing import Optional
 
+from .. import core, helpers
 from ..events import Handler, ValueChangeEventArguments
 from .mixins.value_element import ValueElement
 
@@ -20,6 +21,19 @@ def __init__(self, value: Optional[bool] = False, *, on_change: Optional[Handler
         """
         super().__init__(value=value, on_value_change=on_change)
 
+        # HACK: this is a temporary warning to inform users about issue #3753
+        if core.app.is_started:
+            self._check_for_issue_3753()
+        else:
+            core.app.on_startup(self._check_for_issue_3753)
+
+    def _check_for_issue_3753(self) -> None:
+        if self.client.page.resolve_dark() is None and core.app.config.tailwind:
+            helpers.warn_once(
+                '`ui.dark_mode` is not supported on pages with `dark=None` while running with `tailwind=True` (the default). '
+                'See https://github.com/zauberzeug/nicegui/issues/3753 for more information.'
+            )
+
     def enable(self) -> None:
         """Enable dark mode."""
         self.value = True