To change css go to your admin dashboard
Go to theme edit theme
Select css tab
Write new css in below empty box and save it.
When web page loads it will render the css specified by you.
To know which css to be edit
Go to web page right click on it and select inspect element
Then with select option from inspect element select the area which u want to edit.
After selection u will see the css name of that area and its properties.
Chanhe the properties to whatever u want and write that css in edit css window and save it thats it.
This is process to edit css.
I m noob so i can help this much only.
To modify css as per your needs you need to know the things which is to be written.
Wait for expert help to get css which u can directly paste in edit css box or else try on your own its fun.