Quick and dirty:
import streamlit as st
m = st.markdown("""
<style>
div.stButton > button:first-child {
background-color: rgb(204, 49, 49);
}
</style>""", unsafe_allow_html=True)
b = st.button("test")
More elegant solution is as @andfanilo did by reading the css file and inserting the contents into the style tag.