Zend certified PHP/Magento developer

Custom theme css – less generation issuue – Magento 2 Cloud Commerce

I have an interesting problem. We have a website in Magento Cloud. This site was made by someone before. The problem is this: When we install a new plugin, the plugin’s css files (converted from less files) are not displayed on the frontend. But when we switch from magento panel to defaul theme (luna), there is no problem. The Theme is a custom theme. What could be the problem we are experiencing?

Note: We cannot set developer mode. Magento Cloud does it itself when we push something via git.
We manuelly deleted var/view/process, var/di, var/cache, etc. This does not work when custom theme is active, but it works when we switch to Magento Luma. Please help!