Could you please provide more information on what you are attempting to do as this will help your fellow bloggers in the community to better assist you.

Using Theme > Customize > Advanced  > Add CSS, you can add your custom  CSS code.

If you need to add a print only CSS, you can use a print rule to enclose the print only CSS.

@media only print {
/* Your print only CSS goes here */
}

If you need CSS for mobile devices, you can try the following CSS media queries:

/* Extra small devices (phones, 600px and down) */
@media only screen and (max-width: 600px) {
/* Your CSS goes here */
}

/* Small devices (portrait tablets and large phones, 600px and up) */
@media only screen and (min-width: 600px) {
/* Your CSS goes here */
}

/* Medium devices (landscape tablets, 768px and up) */
@media only screen and (min-width: 768px) {
/* Your CSS goes here */
}

/* Large devices (laptops/desktops, 992px and up) */
@media only screen and (min-width: 992px) {
/* Your CSS goes here */
}

/* Extra large devices (large laptops and desktops, 1200px and up) */
@media only screen and (min-width: 1200px) {
/* Your CSS goes here */
}

Additional information on CSS media queries is available here:
https://www.w3schools.com/cssref/css3_pr_mediaquery.asp
https://www.w3schools.com/css/css_rwd_mediaqueries.asp

If you need a specific CSS code for one post or one page, you can insert CSS code on the relevant post or page by selecting the HTML mode in the Blogger editor:

<style type="text/css">
/* Your page or post specific CSS goes here */
</style>

​I hope this helps you understand the possibilities.

Have a good day.