I don't know if you found this already or not, but I thought I'd post the answer.
/provider/main/welcome.tpl
You can edit it in the "Edit Templates" area.
BTW, you CAN force the webmaster mode to open the "happy box" by clicking the "Open customer area in new window" (in the admin webmaster mode pages after the "happy box" comes up). Once you let it open the customer page change the address bar to provider instead of customer and the "happy box" changes with it.
Hope this helps.