Hello, I updated my theme recently and need to change the color of the category header background from the whitish-gray to something darker, as seen in the screenshot provided. 

There is also now a new bar containing a 'start new topic' button right below the chatbox that I would like to removed in its entirety. How would I go about these tasks? Thank you very much!

Bump

Do you mean to remove all the area of 'bar' or just the 'Start New Topic'

To remove the entire 'bar' then in custom.css add:

.cForumHeader {
  display: none;
}

for changing the background then that would be an image possibly found in your manage resources in admincp (cannot be certain though as depends how image has been added) but if you look through custom.css for:

.ipsType_sectionTitle {
  background: 

you should be able to change it there (as said really depends how has been coded and cannot say for certain without viewing but above should give you a general idea)