Magento 2 give you the ability of starting your theme on top of existing theme or start from scratch , Magento 2 now use the technique of .less and processed it for give you the css result here you will find:

• Create you theme on base of blank theme
• How create your custom  home page
• Where is magento 2 stock all  theme in db
• Change logo
• Change color with variable less (and small explain of how structured)
• Use layout 

command for creating the files :
php bin/magento setup:static-content:deploy

your _theme.less for override the color exist inside this path :
app/design/frontend/Ibnab/abIshak/web/css/source/_theme.less