Make TODO plugin theme aware

Use the theme's colors by default and color the text rather than the
background of items.

Task-number: QTCREATORBUG-17532
Change-Id: I65a2d6da45ce7547b05463a2fb014f8230d0c336
Reviewed-by: Serhii Moroz <frost.asm@gmail.com>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/todo/keyword.cpp

@@ -25,11 +25,12 @@
 ****************************************************************************/
 
 #include "keyword.h"
+#include <utils/theme/theme.h>
 
 namespace Todo {
 namespace Internal {
 
-Keyword::Keyword() : color(Qt::white)
+Keyword::Keyword() : color(Utils::creatorTheme()->color(Utils::Theme::TextColorNormal))
 {
 }