Bug 1554938 - Make WelcomeBox text less thin for readability on low res; r=jlast

Differential Revision: https://phabricator.services.mozilla.com/D32822

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

devtools/client/debugger/src/components/WelcomeBox.css

@@ -12,7 +12,7 @@
   text-align: center;
   background-color: var(--theme-toolbar-background);
   overflow: hidden;
-  font-weight: lighter;
+  font-weight: 300;
   z-index: 10;
   user-select: none;
 }