First, it should be known that there are two different UI's available in Squeak, Morphic and MVC. MVC (Model-View-Controller) is the original Squeak UI, it looks a bit outdated, and is no longer the default interface which Squeak starts up in. Morphic is the newer UI (see the link for details). Most of the following customizations apply for Morphic, but some will also work with MVC. To work in the Morphic environment, see Setting up a morphic project.
Customizing the UI within Squeak
The easiest way to customize Squeak's UI is via the main menu, in MVC or Morphic.
There are several UI appearance changes that can be made from the "appearance..." submenu under the main menu. These include changing the window colors, the screen depth, rounded window corners, full screen mode, etc.
The other important place to look is in the Preferences window, which is under "help..."/"preferences..." or "appearance"/"preferences..." in the main menu. Some key UI preferences include inboardScrollbars (for fixed vs. flop-out scrollbars), scrollBarsOnRight, roundedWindowCorners, and fastDragWindowInMorphic (for dragging a window outline versus a live window). Use the balloon help to learn more about the other preferences.
A goodie which hides scrollbars when there's nothing to scroll is on my web space at http://www.mindspring.com/~dway/smalltalk/. (See HiddenScrollBar26.cs.) Also, in the "notmine" subdirectory are a few UI-enhancing goodies posted by others, including 3dlook-bf.cs, titlebarlook-tb.cs, etc. -Doug Way
The Backdrops goodie by Stephan B. Wessels enhances your options for desktop backdrops using the FileList. With this enhancement you can choose to have your selected image file tiled, stretched, or centered. Or you can use a gradient color backdrop. See http://www.preeminent.org/squeak/. Note that this is pretty old code and has been replaced, generally, by code in V2.8.
Since version 3.2 the Ctrl and Alt text-editing key-combinations may be swapped (cut, paste, etc.) which brings it in line with the MS Windows key shortcut convention. This helps first time users on the Windows platform considerably. Go to Preferences (via the World menu) and choose one of either: "swap control and alt keys" or "duplicate control and alt keys". The balloon help explains it.