Normally, the thumbnails will take up only as much space as they require and the remainder will be allocated to the main image area.
If you wish to bring the thumbnails in a little from the right-hand side of your gallery, increase the stageBorder.
You could also increase the stageHPadding to match and the thumbnails will appear to be horizontally centered in more space than they had previously.
Both the stageBorder and stagePadding configuration options can be found in the General section of the Config Options page.
The thumbnails may not appear to be vertically centered if you use thumbNavPosition="BOTTOM" as space will be reserved (below the thumbnails) for the thumbnail navigation arrows and it is the combination of the thumbnails and the thumbnail navigation arrows that will be vertically centered. Try using thumbNavPosition="CENTER" instead.
If you are unable to layout your gallery as you require with the available configuration options, then try using the Fixed Layout options. These options will give you the ability to determine the size and position of both the main image area and the thumbnail area.
When building a gallery with svBuilder-Pro, the Fixed Layout options can be found in the 'Customize -> Fixed Layout' section and you can see the changes you make, as you make them, in the live preview window.
Also, you may find the SimpleViewer-Pro Layout Guide useful.
SimpleViewer Support Team.