Congratulations on your first plugin! It might be nice to check the interface style on BufEnter, so you can switch to dark mode and have Vim switch the background even if it was already open.
I checked how the darkmode-aware background colors worked the other day, and it seems to be an application default. In custom themes, the only way I found to get toggling background colors is not to configure any. I’ll try doing that in my theme, to see if that’d still look good, sometime soon.
For glare, I usually switch to the light version of my terminal theme. Since Dim uses the same syntax colors for dark and light backgrounds, that just means swapping the text and background colors. I think the easiest way to get a higher contrast would be to use a higher contrast terminal theme.