Bug 1585100 - Make sure text in embedded RDM has dark theme styling r=gl

Overall, the new RDM is using dark theme styling as before. The only exception is the text in the actual toolbar React component, which seems to default to using light theme colors. This revision applies a CSS variable `theme-body-color` to the toolbar's document body to toggle between dark/light theme colors for text.

Differential Revision: https://phabricator.services.mozilla.com/D55706

--HG--
extra : moz-landing-system : lando

devtools/client/responsive/index.css

@@ -46,6 +46,7 @@ body,
 #root {
   height: 100%;
   overflow: hidden;
+  color: var(--theme-body-color);
 }
 
 input:-moz-focusring {