You will need to create your own images in IS's Support\Themes. Banner.jpg [499x58], Banner_mirror.jpg [499x58], preview.jpg [504x386], setup.gif [166x315], Welcome.jpg [499x312] and Welcome_mirror.jpg [499x312]. Mirror images are optional in case of right to left language. Reference Class Theme folder for sample preview.jpg.
Copy Classic.theme from InstallShield's Support\Themes folder and paste it as CustomTheme.theme. Open file in Notepad and change theme name to your custom name and update the path for your images. Then, open InstallShield. New theme should display under User Interface > Dialogs > Themes. Right-click to select.