Set plot backgrounds to white for legibility on dark backgrounds

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: ```sh cargo test --features plotters --features html_reports --test criterion_tests test_output_files ``` [criterion-output.zip](https://github.com/user-attachments/files/22321228/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. ```rust 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: <img width="991" height="761" alt="report" src="https://github.com/user-attachments/assets/979f2064-6668-4671-bd1b-4c24ecc4828f" /> Plotters: <img width="1006" height="758" alt="report" src="https://github.com/user-attachments/assets/c42e8b66-7467-413e-86ca-4514aa0555c6" />