Take a peek at $SPLUNK_HOME/share/splunk/search_mrsparkle/templates/account/login.html - for example, the branding starts at line 130 for me... modify stuff deep down there at your own risk of course.
So how do I insert and center my own logo?
This was clearly explained for v5, now its broken and now explanation on how to fix it.
...and note that any changes you make will be overwritten on upgrade.