@maulana It looks like you've tried to install it from the 'source' not the tar or zip package. As you're on windows I've added a zip package for you. You can download the all in one zip package from the release page. Choose the green button (all in one package), not the grey one (source code).
Have another read of the instructions, it looks like you're trying to install it as a developer rather than the packages that a user would prefer.
The longer answer, is that the source code for the themes does not have the css styles 'built'. The source for bootstrap styles comes as less files which are then 'built' into the final css used in the theme. I've provided gulp build tasks for building the bootstrap less into the final 'default.css' file. The theme also uses twig templates which are installed via composer.
With modern web development tools end users cannot simply install the source and expect it to work. There are dependencies to install (via composer), and processes to run (via gulp) to generate the final set of files that would be used.