is there a way to build my own app template? There are two templates available out-of-the-box: sample app and barebone. I want to create my own template that has some customization in place (like a customized logo etc.).
Please see a similar question, which describes a hack to create a customized app template:
You must create firstly an application that you want it to be your customize app template , and add all the customization that you want that it has .
In splunk web go to "$SPLUNKHOME/etc/apps" folder , copy your app directory and paste it in "$SPLUNKHOME/share/splunk/app_templates" folder .
After doing this restart splunk and go to Apps > Manage Apps > create app Add new panel you will see in drop down template your template app.
For more information about app configurations see Splunk-6.1.1-Developing Views and Apps for Splunk Web