You need to follow the below steps:
Step 1: Create default_head_block.xml like below:
app/design/frontend/[vendor]/[theme]/Magento_Theme/layout/default_head_blocks.xml
Step 2: Andd create local.css like below
app/design/frontend/[vendor]/[theme]/web/css/local.css
Step 3: Add the below code to default_head_blocks.xml
<?xml version="1.0"?> <page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../vendor/magento/framework/Module/etc/module.xsd"> <head> <css src="css/local.css" /> </head> </page>