fix: should be able to see tab focus white box in bottom bar

But actually trigger action for widget under focus need to press
the spacebar in Qt, but we already use spacebar for quit app, so
it might be actually useful once we got a proper shortcut setting
thing.

app/bottombuttongroup.cpp

@@ -24,9 +24,7 @@ BottomButtonGroup::BottomButtonGroup(const std::vector<QAction *> &actionList, Q
                         "background-color:rgba(0,0,0,120)"
                         "}"
                         "QToolButton {"
-                        "background-color:rgba(225,255,255,0);"
-                        "color: white;"
-                        "border-style: none;"
+                        "background:transparent;"
                         "}");
 
     auto newActionBtn = [this](QAction * action) -> QToolButton * {