Documentation CSS: remove body text color

Help text should use the QPalette color.

Task-number: QTBUG-42399
Change-Id: Ibc8658b4a7affc2481d895c13cfa673b50a872e0
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>

doc/global/template/style/offline.css

@@ -2,7 +2,6 @@ body {
     font: normal 400 14px/1.2 Arial;
     margin-top: 85px;
     font-family: Arial, Helvetica;
-    color: #313131;
     text-align: left;
     margin-left: 5px;
     margin-right: 5px;