Allow for customisation of plugin colours

[james-pearce-bath-ac]

It would be useful if the colours for the plugin could be cutomised. It looks like the plugin renders the interface with SVGs with fixed colour values in the SVGs. Also generated graphics for the bar charts are rendered with specific colours. Rather than using hard coded colours it would be useful to have settings for this in the settings screen. This would allow for customisations to match moodle themes and also for accessibility improvements to the graphics.

Currently everything appears to pass WCAG AA but some institutions may need to meet WCAG AAA.

The above changes are not easily achievable with css alone.