Change logo, skin and style in OBIEE 12c
For changing style and skin you should use default OBIEE doc
Customizing the Oracle BI Web User Interface
Quick workaround if you want only change logo
You logo should be near 136x28Update logo in folders
/oracle/product/fmw1221/user_projects/domains/bi/servers/bi_server1/tmp/_WL_user/analytics/za01ic/war/res/s_Alta/master/
/oracle/product/fmw1221/user_projects/domains/bi/servers/bi_server1/tmp/_WL_user/analytics/eiguw6/war/res/s_Alta/master
/oracle/product/fmw1221/user_projects/domains/bi/servers/bi_server1/tmp/_WL_user/bimad_11.1.1/hkbdzw/war/theme/alta/master/
/oracle/product/fmw1221/user_projects/domains/bi/servers/bi_server1/tmp/_WL_user/bipublisher_11.1.1/to5gma/war/theme/alta/master/
/oracle/product/fmw1221/user_projects/domains/bi/servers/obips1/tmp/earmanager/analytics/iNcTF3GuQnj4Ii0d5ex_mg/res/s_Alta/master/
Restart OBIEE and clear browser cache.