CSS styles not working while included in .info.yml file
Working for me creating drupal8 theme using bartik theme as base theme. Successfully call the css file present in my custom theme. Hopes this will helps you. 

name: Smart
type: theme
description: 'Smart business templates'
core: '8.x'
package: Custom
base theme: bartik
project: 'drupal'
stylesheets-remove:
 - colors.css
stylesheets-override:
 - css/colors.css
regions:
 header: Header
 primary_menu: 'Primary menu'
 help: Help
 featured: Featured
 content: Content
 sidebar_first: 'First sidebar'  
 sidebar_second: 'Second sidebar'
 triptych_first: 'Triptych first'
 triptych_middle: 'Triptych middle'
 triptych_last: 'Triptych last'
 footer_firstcolumn: 'Footer first column'
 footer_secondcolumn: 'Footer second column'
 footer_thirdcolumn: 'Footer third column'
 footer_fourthcolumn: 'Footer fourth column'
 footer: Footer

settings:
 shortcut_module_link: false


Also take reference from https://www.drupal.org/node/1876600