Step 1: Reach EcomSolid's Dashboard and press on "Customize" or "Theme Editor".
Step 2: Make sure you are on the page you would like to edit.
Step 3: Locate the "Product" preset section with the product variants and click on it.
Step 4: This will open the settings for this section on the left sidebar. On top of it, click on "Design".
Step 5: Locate the "Color" section and scroll down until you reach "More Settings".
Step 6: Open "More Settings" and locate "Variants Selected Border".
Step 7: Modify the color accordingly to your brand's color and you are all set!