Steps:
From your Shopify admin, go to Online Store -> Themes.
Find the theme that you want to edit, and then click Customize.
Click Theme settings -> General -> Arrow Style
Border Width
Determine the thickness of pagination arrow borders.
Border Radius
Round the corners of arrow. The range is from 0% (arrow border is a rectangle), to 50% (arrow border is a circle).
Width Arrows/ Height Arrows
Determine the width and height of the arrow border.
Size Icon Arrows
Determine the size of the arrow icon inside the border.
Position Horizontal Arrow - (Unit % or px)
Fill in the desired positions of arrows horizontally relative to their slider.