About Customize Viewers

The 'Site -> Customize Viewers' tab allows you to customize the image viewers (gallery types) that Showkase supports: Juicebox, ListViewer and SimpleViewer. Any values changed in this section will be applied as default values when you next create one of these galleries.

Kosel is a theme (rather than a 'viewer'). The theme determines the layout and style of the pages (rather than the galleries that might be embedded in the pages).
If you want to customize a theme beyond what is available in the interface ('Site -> Customize Theme'), then please see the Creating Themes support section.
There's more information there than I could possibly include in a forum post (including a section on 'Inside the _theme Folder' inside the Advanced Theming section which it sounds like you might be interested in).

The _themes/base/ folder contains information common to all themes. The _themes/kosel/ folder contains information specific to the Kosel theme (that builds on the 'base' information). If you can't find what you're looking for in the _themes/base/ folder, try looking in the _themes/kosel/ folder. If whatever you are looking for is directly related to the Kosel theme, then it will likely be in one of these two folders.