ENH: Adding logo image for dark mode

[AakashGfude]

This PR adds the ability to specify a dark-mode version of your logo.
To use a separate logo image for dark mode, simply set it in configuration using the following setting:

html_theme_options:
    dark_logo: logo-image-dark-mode

The image is expected to be in the _static folder of your source repository

[mmcky]

thanks @AakashGfude it looks like this will need some config updates. When you're ready if you could add instructions to the top level description block (and add that the future release notes) that would be amazing. Thanks.

[codecov]

Codecov Report

Merging #231 (812eec1) into master (e65778b) will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #231      +/-   ##
==========================================
- Coverage   73.18%   73.07%   -0.11%     
==========================================
  Files           2        2              
  Lines         261      260       -1     
==========================================
- Hits          191      190       -1     
  Misses         70       70              

Flag	Coverage Δ
pytests	73.07% <ø> (-0.11%)	⬇️

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[mmcky]

@AakashGfude is there any way to adjust the background in the image specified to match the website colour?

maybe the answer is to make a transparent background (cc: @DrDrij)

[AakashGfude]

@mmcky You are right; a transparent background would be good. I can do it with photoshop tomorrow?

[DrDrij]

@AakashGfude Good idea. I've cut out the dark background for transparency. If you send through feedback I'm happy to make another version.

[AakashGfude]

Thanks @DrDrij , the image is working wonderfully now. @mmcky the netlify preview in this PR should have the updated image.

[mmcky]

This is great - thanks @AakashGfude and @DrDrij

[mmcky]

thanks @AakashGfude when you're ready please merge and release.