-Add borders around posts and widgets (thickness and color adjustable), and so I can change the background color without changing the post background (white)
-Shift post and widgets slightly to the left, without changing their widths
and so I can change the background color without changing the post background (white)
I am not sure what you meant by it so please describe it a bit more in detail.
To help us keep support thread separates could you please create new threads for each of your other questions here Activello - Colorlib Support Forum instead of asking them in your single thread as it makes the thread messy and hard to read.
If you want to you can also add reference of this thread in your newly created thread.
We would be more than happy to help you on your new thread.