className property and its sub property

Dear all,

Question 1:
I would like to change color of “subtitle padding” in my header. How can I change

html.H4("Нийт COVID-19-н тохиолдол болон эдгэрсэн хүн", className = "subtitle padded",  style=dict(color = '99CCCC'))

Question 2:
Where should I find className properties like -“subtitle padded”, “sub_page” etc?
It is similar with CSS class and properties but I’ve not got a reliable source with examples.