fix(#4541): fixed transparent background not appearing with dark mode

[Sebastian-Webster]

Fixed transparent background not appearing with chart in dark mode

Fixed the transparent background not appearing when a chart is in dark mode. Previously it would pull the background colour from the dark theme, and now it gets it from the chart options.

Fixes #4541

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes