Fix flickering graphs

[antonymilne]

Description

There's currently a smaller flicker that's detectable when you change theme and then screens. I think I've been able to remove this by changing create_empty_fig to use a transparent background.

Before

Now
Hopefully no flicker.

@petar-qb and/or @nadijagraca please could you give this a careful test to see if (a) you can see the flickering currently and (b) that this has fixed it? I had several failed attempts at fixing this so would be great to double check this one works!

@petar-qb checked and all fixed now.

Also in this PR:

• remove some dead code
• added some more comments just documenting my understanding of the theme update stuff because I found it confusing before

Screenshot

Checklist

• I have not referenced individuals, products or companies in any commits, directly or indirectly
• I have not added data or restricted code in any commits, directly or indirectly
• I have updated the docstring of any public function/class/model changed
• I have added tests to cover my changes (if applicable)

Types of changes

• Docs/refactoring (non-breaking change which improves codebase)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Notice

• I acknowledge and agree that, by checking this box and clicking "Submit Pull Request":

  • I submit this contribution under the Apache 2.0 license and represent that I am entitled to do so on behalf of myself, my employer, or relevant third parties, as applicable.
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  • I certify that the use of this contribution as authorized by the Apache 2.0 license does not violate the intellectual property rights of anyone else.

[petar-qb]

These changes look really good!
I guess there's still a lot of work in purpose to adjust tests here.

[lingyielia]

LGTM! Also tested on my local and server. Works very well 👍