In Google Chrome, go to create a new post. Right click on the post arrow, and click inspect. Then in the inspector pane that opens up, you'll see a button on the top left that toggles the website between mobile view and desktop view. Then, next to that button, there is a pointer you can click - then use that "selector" to hover over the post arrow. You'll see a name at the top of the box that shows up on hover, it's called a CSS Selector. Then you can Google how to modify the attributes of a CSS Selector!
I might have got some nuances wrong, but this should be enough to get you going in the right direction.
You can edit CSS in the Appearances pane of the admin panel.
