I created my own theme which depends on Luma theme. But I have an issue to applies some CSS such _reboot.scss , _grid.scss so how I can remove this?
By default, Magento uses Less for themes styling. It seems you are trying yo compile sass css fiels. You have creat your own theme from scratch and use an alternative preprocessor. Visit below link to configure and use the Gulp task runner and the gulp-sass package for the Sass preprocessor or you can use less css instead of sass.