Plotting using mplfinance uses the default theme instead of our theme

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Description

A clear and concise description of what the bug is.

Steps to reproduce

1. Start up a docker image with deephaven-plugin-matplotlib, mplfinance, and yfinance modules installed:

FROM ghcr.io/deephaven/server:edge
RUN pip install deephaven-plugin-matplotlib
RUN pip install yfinance
RUN pip install mplfinance

2. Create an mplfinance plot:

import matplotlib.pyplot as plt
import pandas as pd
import yfinance as yf
import mplfinance as mpf

TICKER = "AAPL"

plt.figure()
df = yf.download(TICKER, 
                 start="2020-07-01", 
                 end="2020-12-31", 
                 auto_adjust=True)
mpf.plot(df["2020-12-01":])
f = plt.gcf()

Expected results

2. Plot appears with the theme we've defined

Actual results

2. Plot appears but with the default theme, sticking out like a sore thumb:
   

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mplfinance has some additional theming specified, see examples: https://github.com/matplotlib/mplfinance/blob/master/examples/styles.ipynb

Not sure we can set this with just our deephaven-plugin-matplotlib or if we'd need a specific mplfinance one.