[Theme: Illustratr] Changing aspects of Illustratr theme

Hi I am very very new to both wordpress and changing things via css. Any help is greatly appreciated! There are a few changes I would like to make to the Illustratr theme. I apologise if the format of this forum is to ask one question at a time! Still learning how this all works. Thanks for your patience and help, community.

1) I would like to remove or change to white light grey background that appears behind the gallery

2) I would like to make the thumbnails for the gallery larger

3) I would like to remove the heading at the top of each page and the line that appears below it (for example please see the linked sample page, where the heading is “Design” and the black line under it.

4) I would like to remove (or change to white) the light grey bar at the footer of each page

Here is a sample page which includes all of the elements I mentioned above. The site is very much a work in progress.

https://lavanyabalasubramanian.com/wp/index.php/sample-page/design/

Thanks so much for any help, and please keep in mind I am a complete beginner ??