Functions.php

i have 2 themes... the current one has most of the look i'm going for except how the categories are formatted on their page... currently, it shows a default image of a question mark for the featured image if i don't have one assigned... and 4 lines of text from the blog

my other theme i have installed but not activated shows the actual blog on the category page... images and all... and this is how i would like my active theme to be set up...

i know this is a shot in the dark but can i just copy the functions.php from one and paste it in the other? if not, what do i need to find and replace to get what i'm looking for...

thank you