Add basic JupyterLab theme support

[philippjfr]

Ensures that at least the text follows the JupyterLab theme ensuring it is visible in dark mode.

Fixes #520

[codecov]

Codecov Report

Merging #2848 (2fef938) into master (1a334dc) will increase coverage by 0.00%.
The diff coverage is 33.33%.

@@           Coverage Diff           @@
##           master    #2848   +/-   ##
=======================================
  Coverage   83.05%   83.05%           
=======================================
  Files         188      188           
  Lines       24450    24452    +2     
=======================================
+ Hits        20307    20309    +2     
  Misses       4143     4143           

Impacted Files	Coverage Δ
panel/io/notebook.py	52.55% <33.33%> (-0.55%)	⬇️
panel/tests/command/test_serve.py	85.00% <0.00%> (+2.50%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1a334dc...2fef938. Read the comment docs.