Setting the background colour for layer list and legend

[ClarkLangridge]

There are times where, to get the features in a layer to show up against a particular background, the most effective colour to use is white.  The problem is that in the Legend (in web maps and web app builder), table of content, layer list etc. the symbol blends into the background, leave the user unable to tell what the symbol for a feature is:

Being able to set the background colour of the item would fix this problem.