Hi @mauri_mono
If you Applied your css properly and it's not showing changes on frontend then i would suggest you to run below command:
php bin/magento setup:static-content:deploy
if it doesn't work then i think you didn't place css file on correct path for this please check:
Add the following folders to your custom theme
app / design / frontend / [vendor] / [theme] / Magento_Theme / layout
app / design / frontend / [vendor] / [theme] / web / css
put your css on same path inside web/css
once you do this please run the command
php bin/magento setup:static-content:deploy
php bin/magento cache:flush
Hope this will help.
if above answer was helpful then please Accept as Solution & give Kudos
Shubham Khandelwal