Make widget compliant with napari color scheme

[jo-mueller]

Fixes #1

Hi @dstansby ,

I made some changes to the style of the plot widget. Not really by introducing a style sheet (I'm not sure whether that concept applies to canvas/axes objects) but just by creating a ax instance with changed spine/tick/background parameters.

Let me know what you think :)

• passes tests

[codecov-commenter]

Codecov Report

Merging #26 (010a0fc) into main (2f5e92a) will increase coverage by 0.87%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
+ Coverage   89.69%   90.56%   +0.87%     
==========================================
  Files           7        7              
  Lines          97      106       +9     
==========================================
+ Hits           87       96       +9     
  Misses         10       10              

Impacted Files	Coverage Δ
src/napari_matplotlib/base.py	100.00% <100.00%> (ø)

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[dstansby]

Looks great, thanks a lot! I have some suggested improvements in mind, but I'll put those in a new issue since I don't want to block this improvement