Custom CSS not taking effect after a while -Fotografie theme

Good day! Hi everyone. I'm using this free theme that I have customized with some CSS. Now the background is always black, so I have to customize other items that are in black too into white. links, text, dropdowns... specially since i use woocommerce. Then something strange happens... My custom code, i.e. dropdown text and/or background, works in the beginning, then after some time it stops working, that's to say it stops having effect on design. I'm fighting with this issue over some weeks now and it has never happenned to me before. Any idea why this is happenning? Thanks