Every Splunk "app" has its own defined "theme" or style, this is based on images, CSS and JS located in the appserver/static directory.
For example, if you had an app called cisco_searches you could copy those details to (on Linux, assuming default install);
/opt/splunk/etc/apps/cisco_searches/appserver/static
Once you've done that, give Splunk a quick bounce and your cisco_searches app should have a new look (you may need to do a bump or a CTRL+F5 to clear your cache if it still looks the same).
If you want to change all apps to look the same then you will start editing things that you should probably avoid editing, just keep it to an app by app basis.
Some doc detail that provides a more technical explanation if you're up to it 🙂
http://docs.splunk.com/Documentation/Splunk/5.0.3/AdvancedDev/UseCSS
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