CHANGE FONT SIZE & BANNER…PRETTY SIMPLE, RIGHT? NOT FOR ME;0(

Hey, i’m very new to WordPress. i am using Vint-age 1.1 by Dandyna. There are a few simple things i would like to do but can’t figure out:

1. Change the picture of the banner. i was told to name the picture i want to use the same name as the one that is the original in the theme. But when i did that, nothing displayed.

2. i would like to change the color, size and font style of my font. i just want the title of the blogs and my moods to be purple and nice cursive, script like font. i was told to change this in the style sheet, but when i do, the original stuff still displays.

i appreciate any help someone can give me, or if you could point me in the right direction. i tried contacting the person that made the theme. But when i click on their name (which is a link), it takes me to a page that no longer exists.

My site is https://www.aries-raine.com/blog

THANK YOU SO MUCH!!!