Spree Storefront is built on top of Tailwind CSS. After installing Spree you should find 2 new files: You can add or modify styles to the storefront by adding new Tailwind classes to the application.tailwind.css but we recommend creating a new CSS file in the app/assets/stylesheets directory and then importing it in the application.tailwind.css file, eg.

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Importing an additional CSS file

touch app/assets/stylesheets/my_custom_styles.css
in the my_custom_styles.css file, add the following: 
.my-custom-class {
  color: red;
}
and on top of the application.tailwind.css file, add the following: 
@import "my_custom_styles.css";

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Importing a 3rd party CSS library

You can import a 3rd party CSS library directly from a CDN in the application.tailwind.css file, eg. 
@import url("https://esm.sh/[email protected]/swiper-bundle.min.css");

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CSS Variables

Spree Storefront uses a handful of CSS variables that are automatically injected into the body tag. This allows customers to change colors, fonts, etc from the admin dashboard (via page builder). The default variables should be something like this: 
:root {
  --primary: #000000;
  --accent: #F0EFE9;
  --danger: #C73528;
  --success: #00C773;
  --neutral: #999999;

  --background: #FFFFFF;
  --text: #000000;

  --border-default-color: #E9E7DC;
  --border-default-width: 1px;
  --border-default-radius: 6px;

  /* --border-default-shadow: 0px 4px 5px rgba(0, 0, 0, 0); */
  --button: #000000;
  --button-rgb: 0, 0, 0;
  --button-text: #ffffff;
  --button-border-radius: 100px;
  --button-border-width: 1px;
  --button-border-color: ;

  --button-hover: #000000;
  --button-hover-text: #ffffff;

  --secondary-button: #FFFFFF;
  --secondary-button-text: #000000;
  --secondary-button-hover: #000000;
  --secondary-button-hover-text: #FFFFFF;

  --input: #E9E7DC;
  --input-bg: #ffffff;
  --input-text: #6b7280;
  --input-focus-bg: #ffffff;
  --input-border-radius: 8px;

  --neutral-50: #F3F3F3;
  --neutral-100: #EAEAEA;
  --neutral-200: #D4D4D4;
  --neutral-300: #B3B3B3;
  --neutral-400: #999999;
  --neutral-500: #808080;
  --neutral-600: #666666;
  --neutral-700: #4D4D4D;
  --neutral-800: #333333;
  --neutral-900: #1A1A1A;
  --accent-100: #FBFAF8;
}
They are defined in the app/views/themes/default/spree/shared/_css_variables.html.erb file which should be present in your theme.