I attached a Image found on dev.splunk that shows a nice App panel with colors and a cool layout.
I searched on google and Splunk websites on how to change the apps panel layout but I could not find anything related.
Is this a Splunk interface option or should I customize the home app with a css?
That image shows the default home. You can change the order of the apps by dragging and dropping. The colors you see on the left side (by app) are based on each app selected color for its navigation menu bar. This can be changed on navigation xml per app basis.