Magento2 -- To change the theme css and colors in Magento2

To change the theme css in magento2


   create a file
   app/design/frontend/Magento/<theme-frontend-sample>/web/css/source/_extend.less 
add respective changes in above file


Example to override the color variables
      @color-orange-red1: #ff5501; change to
   @color-orange-red1: #FF3645;
it is overrides all orange color to your needed one.

Same way you can override the classes.

.action.primary {
    background: #000000;
  } 

For more information

http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/css-guide/css_quick_guide_mode.html