Changing dark mode makes paint dirty

[JamesHollyer]

This is a fix for issue 5246.

Upon initial change from light to dark mode and dark to light mode the graphs used to look like this:

and now they look like this:

The problem was that that change from dark to light mode did lot indicate the the SeriesLineView that the lines were "dirty" so upon update it did not realize they needed to be redrawn.

Fixes #5246.

[stephanwlee]

Ah, great. I did not think this would literally be one line fix. Would you mind taking a look at the test and meaningfully prevent regression or detect a bug? (A possible answer is, it is hard to test this in a way that can prevent any regressions and require more integrational style testing such as one that uses WebDriver).

Also, pro-tip: if you type "Fixes #<ISSUE_NUMBER>.", GitHub will automatically close the issue when you merge the PR (https://docs.github.com/en/github/writing-on-github/working-with-advanced-formatting/using-keywords-in-issues-and-pull-requests).

[stephanwlee]

In case testing this is more challenging than adding a few LOC, I am approving the change.