I wanted to ask if it is possible to use stock icons instead grabbing icons from here and there and having mess appearance Try zooming attachment for example to quickly notice this issues
I think that it will be done in the future.
It seems that DC does not render with alpha channel, and some of icons in screenshot below look completely different then shown in DC. For example, I attached image of first icon (refresh) that's linked to the toolbar
GTK2 version of DC don't supports alpha channel in icons on toolbar (and also in some places), it is a Lazarus limitation. I think it will be fixed in future.