Expand Add Nodes Toggle Lacks Hover Styles when "off"

[sbates-idrc]

The expand add nodes toggle has hover styles for all themes when it’s "on". It lacks hover styles when it’s "off" for mixed/light/dark themes. It has hover styles for greyscale/contrast themes when it’s "off".

Original Jira issue: https://fluidproject.atlassian.net/browse/C2LC-563