app.py

import gradio as gr

customjs = """(mos) => { window.location.href = '?__theme=' + mos + '' }"""
mylist=["dark","light","system"]
def get_thm(bork,request: gr.Request):
    selected = request.query_params.get("__theme", ""+bork+"")
    return gr.Dropdown(choices=mylist,value=""+bork+"")
with gr.Blocks() as iface:
    
    mes=gr.Dropdown(choices=mylist,value=mylist[0])
    btn=gr.Button("select theme")
    btn.click(None, mes, None, js=customjs).then(
    get_thm, [mes], [mes])
    iface.load(get_thm, [mes], [mes])
iface.launch()