Adjust custom themes default naming

 - Change custom themes default naming, use custom themes amount plus 1
 as naming not "Custom", Doc: https://docs.google.com/presentation/d/15JadcGi5k1_0znUN_XdFpU7UxeXwxjK3Y7LxvhW5ETM/edit?ts=5eb540b5#slide=id.g812429ed29_51_16
 Video: https://drive.google.com/file/d/1Ijin203kGq9D9AG5FogaNtmb2gkcGfcW/view?usp=sharing

 Bug: 151285666
 Test: Manually

Change-Id: Ief25203a7911617d4f6287a44e2833db55ca8b2b