Hello,
Please add this code to the Theme Settings > Custom CSS > Global:
body .labels-rounded .onsale {
background-color:#51AA1B;
}
You can change the font by means of Advanced typography option, which allows to choose the item from a drop-down or insert your custom CSS class. Please navigate to the Theme Settings > Typography > Advanced Typography. You will find the product price on the product page and in the grid.
Here is the documentation providing more detailed instructions: https://xtemos.com/docs/woodmart/advanced-typography-settings/
Best Regards