Changing Add to Basket button (colour/ shape)

Greetings! I wish to alter the button's appearance by making it black and slimmer. Regrettably, I cannot seem to find the formatting option for the button. I was only able to personalise the text, but not the colour or shape. Additionally, I would like to align the button with the quantity box since the "Add to basket" button seems to be higher than the Qty. Is there anything I may have overlooked? I would highly appreciate your support in resolving this issue. Kindly refer to the attached images of the present button and the desired black button. Thank you very much!

Hi,

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