Following on from my previous post I have been advised to create an idea for this issue so here we go...
For web maps which have points classified with multiple custom svg symbols, and aggregation turned on, it would use useful if you could customise the colour of the symbol used to show aggregation in the legend. At present, even if all the svgs are black, the aggregation symbol shows in a light grey with no option to change it.

The contrast for this grey colour is poor against the white legend and fails WCAG standards (as shown below using WebAIM Contrast checker). It also does not reflect the actual colour of the symbol used in the map.

Furthermore, customisation would be even more helpful when you embed the map in a dashboard or EB with a custom background colour as in those cases you may have different needs (e.g. a lighter symbol against a darker background).
As an aside, it would be nice to have more customisation in general for the legend with regard to aggregation. I know the fact that non-integer values displaying when the data is discrete is something that has been raised before and I agree this is confusing.