Great question — answering as the UphillTech team:
First you need to use color instead of colour . You should try this css: .woocommerce-page #content .cart-collaterals .cart_totals>table tr.total .includes_tax .amount { font-size: 18px !important; color: #000 !important; } This should work.
Full answer by LoicTheAztec: https://stackoverflow.stackexchange.com/a/42393158 (CC BY-SA 4.0).
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