(Please ask one question per thread.)
[ol][li]If we're talking about folders launched from outside of Opus (e.g. double-clicking the Computer icon on the desktop) then you just need to turn on Preferences / Launching Opus / Explorer Replacement / Open external folders in a new tab.
If we're talking about opening folders in another way, please give us the details.
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[li]Do you want to change the icons of all folders, or only some?
You can use Preferences / Favorites and Recent / File and Folder Labels to do this, either way. Exact steps depend on which you want. If you want all folders you could create a wildcard label that matches * and only changes the icon color.[/li][/ol]