Step 1: Log in to WHMCS Admin
Step 2: Go to Add-ons > Vue Js Whmcs Cart >Setting

Step 3: Click on setting then go to the general setting then you can hide/unhide override css over cart and click on the Save button

Step 4: Open FTP and go on this path "/order/assets/styles/overrides/" [Order folder should be your vue cart folder]
Step 5: You will see the following file

Step 6: Rename .new from this file so that the file should be overrides.css
Step7: Now you edit the file and write your own CSS rule to override the existing CSS rule
Step 8: Now your file will be displayed like this
