I added bootstrap 4 to my site, but it overwrites many magento 2 core css.

So I added a new file THEME\web\css\bootstrap.less where I try to fix these problems, and added it to my THEME\Magento_Theme\layout\default_head_blocks.xml.


body {
    font-family: 'Open Sans','Helvetica Neue',Helvetica,Arial,sans-serif !important;
    font-size: @font-size__base;
    line-height: 1.42857143;

.nav { display: block; }

But if I compile then I get NameError: variable @font-size__base is undefined in pub/static/frontend/company/fresh/de_DE/css/bootstrap.less on line 3, column 16:, because I am using the variable @font-size__base which is defined in lib\web\css\source\lib\variables\_typography.less

How can I use magento less core variables in my custom css file?

My theme inherits from magento luma.

1 Answer 1


Instead of creating new bootstrap.less file, please create _bootstrap.less file in <your_theme>/web/css/source/extends directory.


  1. Create the _extend.less file in <your_theme>/web/css/source/

  2. Import your _bootstrap.less in this file:

    @import 'extends/_bootstrap.less';

  3. Make sure that your theme inherits from Magento/luma or Magento/blank theme - this is necessary if you want to use Magento variables

  4. Recompile your styles

I hope this helps you. Thanks, Marcin

  • Thank you, but this will compile my bootstrap.less into the files styles-l.css and styles-m.css which both get loaded before the bootstrap 4 css file. This means that my overwrites will get overwritten again by bootstrap 4 ... So I need a separate css file which gets loaded after the bootstrap 4 css file.
    – Black
    Commented Aug 27, 2020 at 14:00

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