Bug
1642376: Move inline style for panel popups into each theme's popup.css. r=dao
This supports the fact that devtools, which is hosted in an "about:" page,
does not use inline styles. They are prevented from working by the CSP defined
at devtools/client/framework/toolbox.xhtml. This patch moves The styles that
are added by the MozPanel.initialize method into the popup.css files for each
theme, where the other .panel-arrowcontent class selectors live.
Differential Revision: https://phabricator.services.mozilla.com/D78832