Tooltip and Combobox bugs with Breeze / Dark theme

setStyleSheet was overriding default tooltip theme from the system,
causing themes like Breeze to render white text on a grey bg for
tooltips. Tooltips now render default for the system theme everywhere.

set a theme matching color on the dark theme comboboxes.

fixes #118

src/CommunitiesList.cpp

@@ -16,8 +16,6 @@ CommunitiesList::CommunitiesList(QWidget *parent)
         sizePolicy.setVerticalStretch(1);
         setSizePolicy(sizePolicy);
 
-        setStyleSheet("border-style: none;");
-
         topLayout_ = new QVBoxLayout(this);
         topLayout_->setSpacing(0);
         topLayout_->setMargin(0);