Fix theming on HoloViews plot updates

[philippjfr]

Fixes #1816
Fixes #2207

[hoxbro]

I just tested, and this does also fix #1816.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@c84ee56). Click here to learn what that means.
The diff coverage is 80.00%.

@@            Coverage Diff            @@
##             master    #2209   +/-   ##
=========================================
  Coverage          ?   83.81%           
=========================================
  Files             ?      182           
  Lines             ?    21789           
  Branches          ?        0           
=========================================
  Hits              ?    18263           
  Misses            ?     3526           
  Partials          ?        0           

Impacted Files	Coverage Δ
panel/template/base.py	78.14% <66.66%> (ø)
panel/pane/holoviews.py	79.95% <100.00%> (ø)

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[philippjfr]

It does indeed.