Regarding:
Updated info for version 1.23 (probably for some older versions too):
1. In the viewer, turn on the Advanced menu by pressing Ctrl-Alt-D.
2. Select Advanced -> Debug Settings...
3. In the drop-down, type SEL. This will auto-scroll the drop-down to the SelectionHighlightAlpha option.
4. Enter a value (0.0 = completely transparent, 1.0 = completely opaque).
5. Restart the Second Life viewer.
Updated info for version 1.23 (probably for some older versions too):
1. In the viewer, turn on the Advanced menu by pressing Ctrl-Alt-D.
2. Select Advanced -> Debug Settings...
3. In the drop-down, type SEL. This will auto-scroll the drop-down to the SelectionHighlightAlpha option.
4. Enter a value (0.0 = completely transparent, 1.0 = completely opaque).
5. Restart the Second Life viewer.
Very helpful