The quickest way to change the appearance of your viewer is to modify the Toolbar Spritemap.

You can download the default spritemap here:

You can change the look and feel of the buttons as long as you do not change the size and position.

You then upload your new spritemap using file manager and add the following line to the HeadCSS in Viewer Config:

#col1content li a, #col2content li a, #col2content li .slidebar_rollover
{ background-image: url(""); }

If you need to modify the background color of the two columns, add the code below and modify the hex code as desired:

#col2content ul, #col1content ul {
  background-color: #FFFFFF;