Hi!
I have fair, though a bit outdated, CSS knowledge, but I’m in no way a designer.
To create a simple theme with only CSS overrides, what should the starting point(s) be? I’m not talking about tools (discourse_theme and stuff like that), just purely from a design perspective.
A good example of what I’d like to do is redesigning the navigation menus (nav-pills).
Each time I’ll achieve something, it will be “break” something elsewhere, because the contexts in which the modified elements lie are different.
If I change their dimension through padding, they won’t be the same height as adjacent buttons (new topic) or selectors (category, tags) and there will be a discrepancy.
If I add a border to my pills, it will look ugly on the user preference pages because the navigation bars are already separated with borders.
I also tried to add some modifications to select-kit and it appeared that it would be a nightmare to have my modifications right for any select-kit variation.
I have issues pretty much every time I try to redesign an element.
So, I suppose starting a redesign from a given page is not the right solution. I thought of starting from the /styleguide page since it’s probably its purpose, but it’s easy to notice that the problem is often quite the same. Each element displayed in styleguide lacks the context in which it is used on a real page.
There are so many elements and interactions between elements and pages that it’s quickly overwhelming and I start to exponentially patch my modifications to “fix” them, which is exactly what I shouldn’t do.
I hope my problem’s description isn’t too vague.
If you have any advice to share that would help me make my small changes, that would be much appreciated! 
My question is quite broad, but to show what kind of modifications I aim to make, here’s the default theme of my forum, and my current work.
Visually, it won’t be much more than what is displayed and this page looks good, but currently, there are too many areas on other pages that are glitchy or ugly because of my current tweaks (and more I haven’t yet discovered, of course).