How to edit HTML?

Hello everyone!
I’ve got 2 problems to deal with.

First problem
I have an issue with my site’s footer. For some reason whenever I activate a certain plugin the background continues under the footer. I tried contacting the author for days but he hasn’t helped yet.

Anyway, it does this only on the main homepage and not anywhere else on the whole site. Here’s how it looks like:

https://s17.postimg.org/ezdd6xiun/ft2.jpg

I think I may have found what’s wrong though. I checked with Chrome’s Developer Tools and in the homepage it says custom-background only-on-home. In the other pages it only says custom-background.
Here’s a screenshot, yellow highlight:

https://s29.postimg.org/fn1tfejl3/custom_background.jpg

I guess this is in HTML though, how could I edit and alter this line in WordPress so I can remove the “only-on-home”?

Second problem
I added SSL to my site so it’s https now (Cloudflare). However, in Developers Tools it says that a certain plugin was detected requesting an insecure stylesheet. So I click it and it goes to (index) in the Sources tab of Developers Tools. It shows me that the below request has http instead of https:

https://s17.postimg.org/dpcu767xr/qwert.jpg

So I suppose if I manage to edit it and replace http with https this will be solved as well. This shows HTML though as well. My question is, which file contains all this information displayed in Developers Tools so I can edit it and add the https?

I’ve been searching for so many hours!

I would be grateful for your help!

Thank you.