Use ES6 template strings

src/qml/Pages/AddChat/EncryptCheckBox.qml

@@ -6,7 +6,7 @@ HCheckBox {
     subtitle.text:
         qsTr("Protect the room against eavesdropper. Only you " +
              "and those you trust can read the conversation.") +
-        "<br><font color='" + theme.colors.middleBackground + "'>" +
+        `<br><font color="${theme.colors.middleBackground}">` +
         qsTr("Cannot be disabled later!") +
         "</font>"
     subtitle.textFormat: Text.StyledText