Sorry, where is your question? You can edit your child theme’s style.css file to add the CSS rules you want.
FYI, ATUM settings also offer an option to customize the colours shown in the ATUM Interface. Please, read this article. However, I don’t think these settings can help you with what you want.
I think it would be better for you to contact a developer to assist you. You don’t need the HTML tags to apply the CSS (indeed, it isn’t recommended to use them).
I include the previous CSS selector with its HTML tags, but please, note that it may not have enough ‘weight’ to change what you want. You should be able to get the HTML entities and add more ‘weight’ to the selector by inspecting the Stock Central’s HTML code.
