You can follow this instruction for How to show Quantity Selector & Add to Cart Button in Same Line For Shopify Dawn Theme and other free 2.0 Shopify's themes. Please follow these steps.
- Login to your Shopify store
- Online store > Themes > Edit code
- Open base.css file
and paste these codes below
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nav.header__inline-menu { text-align: center; }
@media screen and (min-width: 990px) {
.header--middle-left { grid-template-areas: "heading navigation icons"; grid-template-columns: 1fr 1fr 1fr; column-gap: 1rem; }
}