Unable to switch theme when using panel with fastapi

[ludwigHoon]

ALL software version info

(this library, plus any other relevant software, e.g. bokeh, python, notebook, OS, browser, etc should be added within the dropdown below.)

Software Version Info

```plaintext python 3.14.3 bokeh 3.8.2 bokeh_fastapi 0.1.6 panel 1.8.9 uvicorn 0.41.0 ```

Description of expected behavior and the observed behavior

When visiting the site at 127.0.0.1:8000/panel, I should be able to toggle the dark mode, but it is not changing. When visiting 127.0.0.1:8000/panel?theme=dark, no error is created, but the dark mode is not toggled.

Complete, minimal, self-contained example code that reproduces the issue

import hvplot.pandas
import numpy as np
import panel as pn
from fastapi import FastAPI
from panel.io.fastapi import add_application

import pandas as pd

xs = np.linspace(0, np.pi)

freq = pn.widgets.FloatSlider(name="Frequency", start=0, end=10, value=2)
phase = pn.widgets.FloatSlider(name="Phase", start=0, end=np.pi)

def sine(freq, phase):
    return pd.DataFrame(dict(y=np.sin(xs*freq+phase)), index=xs)

def cosine(freq, phase):
    return pd.DataFrame(dict(y=np.cos(xs*freq+phase)), index=xs)

dfi_sine = hvplot.bind(sine, freq, phase).interactive()
dfi_cosine = hvplot.bind(cosine, freq, phase).interactive()

plot_opts = dict(responsive=True, min_height=400)

# Instantiate the template with widgets displayed in the sidebar
template = pn.template.FastGridTemplate(#.EditableTemplate(
    #editable=True,
    title='EditableTemplate',
    sidebar=[freq, phase],
)

template.main[:2, :5] = dfi_sine.hvplot(title='Sine Plot', **plot_opts).output()
template.main[:2, 7:] = dfi_cosine.hvplot(title='Cosine Plot', **plot_opts).output()

app = FastAPI()

@add_application('/panel', app=app, title='My Panel App')
def create_panel_app():
    slider = pn.widgets.IntSlider(name='Slider', start=0, end=10, value=3)
    return template #slider.rx() * '⭐'

if __name__ == '__main__':
    import uvicorn
    uvicorn.run(app)

Stack traceback and/or browser JavaScript console output

Exception in callback functools.partial(<bound method IOLoop._discard_future_result of <tornado.platform.asyncio.AsyncIOMainLoop object at 0x3063ab0e0>>, <Task finished name='Task-65' coro=<ServerSession.with_document_locked() done, defined at /Users/ludwig/Documents/c2/.pixi/envs/default/lib/python3.14/site-packages/bokeh/server/session.py:77> exception=RuntimeError("Unexpected ASGI message 'websocket.send', after sending 'websocket.close' or response already completed.")>)
Traceback (most recent call last):

Screenshots or screencasts of the bug in action

• I may be interested in making a pull request to address this