How do you change the font size of a markdown only for the mobile version?

I've got a website that has custom markdown code for the title of each page, but the font is too big in the mobile and tablet versions. How do I customize the size of the font to fit mobile and tablet versions? Thanks!

You should share site url

or if you know a bit html/css, you can use query

@media screen and (max-width:900px) {
paste css to decrease font here
}