Perhaps this change is already something for SolarWinds' development team and is not customizable, unless you try looking into the web files if you can change it there. I suggest you highlight this as a feature request for the next release.
For the active response "send popup message", we're just using windows default popups. You can customize the message contents (the text string that appears in the message), but I don't think it has capabilities to do much more than that. We might be able to use something a little more fancypants and make it a little more customizable. Definitely submit an idea/feature request and we'll look into this one.