I downloaded Suitecrm 8.4.0 from git and css is not working for this interface
You probably have files with the wrong permissions or owned by the wrong user.
to fix files and folders permissions, run this on the SuiteCRM root folder:
sudo find . -type d -exec chmod 755 {} \; && sudo find . -type f -exec chmod 644 {} \;
To ensure all files are owned by the correct user:
sudo chown -R apache. ./
My user in eg is apache. In EL8 ≥ if the user and group is the same you can specify only user. and it will apply to both user and group.
Alternative complete command:
sudo chown -R apache.apache ./