Ensure that template.config.raw_css is correctly applied

[philippjfr]

Fix #3562

[codecov]

Codecov Report

Merging #4018 (5e46eea) into master (081d13b) will decrease coverage by 9.00%.
The diff coverage is 31.25%.

@@            Coverage Diff             @@
##           master    #4018      +/-   ##
==========================================
- Coverage   83.65%   74.64%   -9.01%     
==========================================
  Files         223      223              
  Lines       33132    33148      +16     
==========================================
- Hits        27717    24744    -2973     
- Misses       5415     8404    +2989     

Flag	Coverage Δ
ui-tests	?
unitexamples-tests	74.64% <31.25%> (-0.04%)	⬇️

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Impacted Files	Coverage Δ
panel/template/base.py	76.64% <ø> (ø)
panel/tests/ui/template/test_bootstraptemplate.py	30.23% <31.25%> (-69.77%)	⬇️
panel/tests/ui/widgets/test_tabulator.py	9.59% <0.00%> (-89.38%)	⬇️
panel/tests/ui/widgets/test_input.py	11.34% <0.00%> (-88.13%)	⬇️
panel/tests/ui/layout/test_accordion.py	14.72% <0.00%> (-85.28%)	⬇️
panel/tests/ui/widgets/test_sliders.py	15.42% <0.00%> (-84.58%)	⬇️
panel/tests/ui/layout/test_card.py	18.01% <0.00%> (-81.99%)	⬇️
panel/tests/ui/io/test_convert.py	15.92% <0.00%> (-79.65%)	⬇️
panel/tests/ui/test_reactive.py	26.53% <0.00%> (-73.47%)	⬇️
panel/tests/ui/template/test_fastgridtemplate.py	28.57% <0.00%> (-71.43%)	⬇️
... and 32 more

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