How to customize bootstrap theme non-destructively?

[johnywhy]

It is not bootstrap.

The mains css is a bit daunting at 3800 lines but make a new heading at the bottom of it 
/*johnywhy's edits*/
keep copying and saving with a new filename and you can always go back.

If you don't want to actually alter the css as provided it should be enough just to copy and paste to the bottom the declarations you want to change and as long as they are after the originals they will take precedence. It won't make for a small file but that file is totally bloated anyway so it won't make much difference.

Timbow, thx for details! You're right, it's not bootstrap. 
http://get-simple.info/extend/theme/spectral/1059/

I'd rather not put any custom CSS into the theme's stylesheet, not even at the bottom. Want my css to be a separate file. I assume i can put an @import statement at the bottom (or top) of their css, to include my file. 

re the bloat, yeah that sucks, but my site loads super-quick, so it's apparently not an issue.

However, i noticed that when i apply css to the theme in my browser dev pane, it works fine. But the same css manual added to the theme's stylesheet does not work, or works differently. Not sure how to do proceed. 

thx!