Is there any way of replacing the standard workbench scroll bars?

There are a number of options, Birdie replaces the plain looking window border with any image you want. It can be downloaded from http://www.vgr.com/birdie. Alternatively try Visual Prefs or MCP from Aminet. These do require some configuration. It is advised to try one option at a time. Birdie requires a fairly fast processor to run well.

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