Set plot backgrounds to white for legibility on dark backgrounds

[rben01]

This PR sets plot backgrounds to white in both the gnuplot and plotters backends. I'm not sure how to test this, so for verification, attached are the reports you get when running the following command with and without gnuplot installed:

cargo test --features plotters --features html_reports --test criterion_tests test_output_files

criterion-output.zip

The reports are saved to a tempdir, so if you would like to replicate this yourself, you should place the following at the top of the test function and then add -- --nocapture to the command above.

let mut tempdir = temp_dir();
tempdir.disable_cleanup(true);
eprintln!("{tempdir:?}");  // copy this and cd to it in another shell

Screenshots of the reports in dark mode:

Gnuplot:

Plotters:

[rben01]

I think the horizontal blue line in the second screenshot is an artifact of having had web inspector open and not inherent to the output.