Animated gif as publication cover - size optimisation

Hello there, kinda new to Ghost. I started running my blog https://cruising-collapse.ghost.io/ in September.

I recently asked a friend to design an animated gif to have a customised publication cover, but it seems that Ghost - or at least the theme I chose, Casper - is having trouble adapting the gif size.

Although it seems to work on mobile, apparently the reader needs to zoom the web page to 133% to get the full image. I tried uploading the gif in three different sizes (888x500, 1280x720, 1500x844) but it always ends up cropping the cover. I also tried some code injection I found, i.e.

<style>
.kg-image-card img {
  height: auto;
}
</style>

but it doesn’t seem to help much.
I’ve read that I should try to solve the issue running Ghost remotely but I’ve had troubles installing the correct Node.js version and I don’t think I have the right skills to compile the code manually.

Can anybody give me a hand please? Thanks a lot!

You probably just need to set object-fit: contain on the image, using css and code injection. If you want to link the site, someone here can probably help with selectors if needed.