ahh got it! i'd actually been meaning to make a new post with a 1-column theme variation but if you don't want to have any posts at all and only need the sidebar centered horizontally then let me see if i can fiddle around with that for you here :D

+ ok so there are a few ways of doing this but i wanted to find a fairly simple and painless way to get to it, so if you just plop this anywhere in your code box it should work !!! if you have any tweaks you want with padding, resizing etc. let me know but hopefully this looks ok!

#content>.inner {
display: flex;
align-items: center;
margin: 0 auto;
flex-direction: column;
}

#secondary {
max-width: 500px; [change this to what you want your sidebar's max size to be]
}