Improve handling of stylesheet URLs

[philippjfr]

No description provided.

[codecov]

Codecov Report

Merging #4540 (9e49750) into main (bf32f23) will increase coverage by 0.08%.
The diff coverage is 95.16%.

@@            Coverage Diff             @@
##             main    #4540      +/-   ##
==========================================
+ Coverage   83.51%   83.60%   +0.08%     
==========================================
  Files         250      252       +2     
  Lines       36542    36647     +105     
==========================================
+ Hits        30518    30638     +120     
+ Misses       6024     6009      -15     

Flag	Coverage Δ
ui-tests	39.52% <47.95%> (+0.03%)	⬆️
unitexamples-tests	73.65% <95.16%> (+0.12%)	⬆️

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Impacted Files	Coverage Δ
panel/compiler.py	11.60% <20.00%> (+0.81%)	⬆️
panel/io/server.py	74.77% <50.00%> (-0.10%)	⬇️
panel/util/__init__.py	87.55% <71.42%> (-0.49%)	⬇️
panel/template/base.py	77.97% <78.57%> (+0.28%)	⬆️
panel/theme/base.py	89.01% <89.28%> (-0.55%)	⬇️
panel/io/resources.py	82.35% <100.00%> (+0.40%)	⬆️
panel/layout/card.py	93.75% <100.00%> (+0.09%)	⬆️
panel/layout/grid.py	75.74% <100.00%> (+0.06%)	⬆️
panel/layout/gridstack.py	85.07% <100.00%> (ø)
panel/layout/spacer.py	79.31% <100.00%> (+2.38%)	⬆️
... and 16 more

... and 2 files with indirect coverage changes

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