The details shared here apply to the most recent versions of our themes. To keep your theme up to date, check out this helpful document.
By default, the "Add to cart" buttons in your theme have its background color determined by the "Buttons" color setting under Theme Settings > Colors.
If you have the Flex theme, this setting is found in Theme Settings > Buttons > "Add to cart" Button.
However, if you choose to enable Shopify's "dynamic checkout buttons" feature, you'll notice a slight change, where the "Buy it now" button takes over the "Add to cart" button's styling:
When the dynamic checkout buttons are enabled, the styling comes from Shopify. The rationale behind the color change is that if the dynamic checkout buttons are enabled, the emphasis should shift to that button instead, which is why the "Add to cart" then inverts and becomes an outline style, in order to de-emphasize it.
If the "Add to cart" button were to stay the same, it would 'fight' for attention with the "Buy it now" button.
If you'd like the "Add to cart" button to keep it's default color styling and be the main call to action, the best advice is to disable the "dynamic checkout buttons" on your shop.
Alternatively, if you're familiar with CSS, you can override the settings by adding in a custom code. Please be advised that this is not within our scope of support, and if you require hands-on assistance we suggest reaching out to a Shopify Expert.