A CompStore theme inherits the Luma theme, which in turn inherits a blank theme, as shown here:

Once you have to make changes in CompStore in order to customize the new theme, you can think about the functionalities already available in the other themes to apply your changes.
The vendor directory under the Magento 2.0 root directory handles all the native Magento modules and themes. The Magento blank and Luma themes, which you have been working on until now, are available in vendor/magento/theme-frontend-blank and vendor/magento/theme-frontend-luma, respectively. So, the CompStore theme "receives" all the features of the themes under these folders. It's important to fix these basic concepts to understand the context that you inserted when you developed a Magento theme solution.
Once you have a solid concept about the behavior, let's create a custom .css file for the CompStore theme:
packt/vendor/magento/theme-frontend-blank/web/css/_styles.less file to the packt/app/design/frontend/Packt/compstore/web/css locationimport command as the following example:@import 'source/lib/_lib.less'; @import 'source/_sources.less'; @import 'source/_components.less'; @import 'source/compstore.less';
compstore.less file under the packt/app/design/frontend/compstore/web/css/source directory and type this code:@color-compstore: #F6F6F6;
body{
background: @color-compstore;
}_theme.less file under the packt/app/design/frontend/compstore/web/css/source directory. Execute the following://Change color of elements in Product Page @color-catalog: #4A96AD; @page__background-color: @color-catalog; @sidebar__background-color: @color-gray40; @primary__color: @color-gray80; @border-color__base: @color-gray76; @link__color: @color-gray56; @link__hover__color: @color-gray60; @button__color: @color-gray20; @button__background: @color-gray80; @button__border: 1px solid @border-color__base; @button-primary__background: @color-orange-red1; @button-primary__border: 1px solid @color-orange-red2; @button-primary__color: @color-white; @button-primary__hover__background: darken(@color-orange-red1, 5%); @button-primary__hover__border: 1px solid @color-orange-red2; @button-primary__hover__color: @color-white; @navigation-level0-item__color: @color-gray80; @submenu-item__color: @color-gray80; @navigation__background: @color-gray40; @navigation-desktop-level0-item__color: @color-gray80; @navigation-desktop-level0-item__hover__color: @color-gray34; @navigation-desktop-level0-item__active__color: @navigation-desktop-level0-item__color; @tab-control__background-color: @page__background-color; @form-element-input__background: @color-gray89; @form-element-input-placeholder__color: @color-gray60; @header-icons-color: @color-gray89; @header-icons-color-hover: @color-gray60;
With the compstore.less and _theme.less files, the background and product page colors will change according to the new proposal of the CompStore theme.