I did create custom.css in app/design/frontend/hsc/hsc-theme/web/css. But it didn't work how to do it? How to override default css in new theme in magento 2.1?
To customise the CSS you simply override the .LESS file where the original styles you want to change exist, using the equivalent name/path in your own theme. So for example if you wanted to override the checkout CSS, originally defined in the following file: app/design/frontend/Magento/blank/Magento_Checkout/web/css/ source/_module.less
You would copy the contents of this file and create a new file in your own theme with the same path. So, it would look something like this: app/design/frontend/hsc/hsc-theme/web/css/source/_module.less
Also make sure your theme is activate from Magento 2 backend Content > Design > Themes.