Helo @elpanda13,
Thanks for updating the thread. 
I can confirm the issue on my end. On desktop changes are working fine but they are not getting reflected on mobile devices. Upon further investigating the issue I see that the media queries codes you have added is not targeting the mobile devices. I see that code is being added in page css editor. Open the page in content builder and click on CSS to see the code. You need to adjust the view port side to make the necessary changes. Because code is not geared towards smaller viewports, those elements are pulling Theme CSS. A sample code for mobile devices in you case would look like:
@media only screen and (max-width: 600px) {
.pt-acc.gray .x-accordion-heading .x-accordion-toggle.collapsed, .pt-acc.gray .x-accordion-heading .x-accordion-toggle, .pt-acc.gray .x-accordion-inner {
color: #3d3f43;
border-bottom: 1px solid rgba(255,255,255,0.3);
font-family: "Anton",sans-serif;
background-color: #d7d7d5;
}
}
I am also sharing few relevant tutorials that will help you to get started with media queries.
https://www.w3schools.com/css/css_rwd_mediaqueries.asp
If you would like to learn CSS, I am sharing few resources that you take a look to get started with CSS and an interesting tool that you can use to speed up the development process.
I recommend you to watch following video that will help you to get started with CSS.
https://www.youtube.com/watch?v=MFR4WXiLzpc
Sometimes it can get a bit difficult to find out the right selector to be able to write the required CSS codes. A handy tool that can help you in this is Google Chrome dev tools. I am sharing the resource that you can refer to get started with dev tools.
https://developers.google.com/web/tools/chrome-devtools/css/
https://developers.google.com/web/tools/chrome-devtools/
https://www.youtube.com/watch?v=tP_kXBJWPhQ&t=200s
Thanks.