Steps

  1. Locate the default theme
    bundle show minima
    
  2. It will show C:\Ruby\lib\ruby\gems\2.5.0\gems\minima-2.5.0. The theme’s directory will be:
    ├─assets
    ├─_includes
    ├─_layouts
    └─_sass
     └─minima
    
  3. Override the default theme. If you want to override the home page, create a _layouts folder into your site directory. Ex.my_site_layouts. Then copy C:\Ruby\lib\ruby\gems\2.5.0\gems\minima-2.5.0_layouts\home.html to my_site_layouts. Edit home.html. Modify via RSS as via something. modify-theme

Reference

themes|JekyII