CSS specificity: make your CSS selector more specific, because I’m not sure which one would count as being more specific. Easiest one would be .job-conf > .light-btn.btn-primary:not(.btn-link):not(.btn-flat) – that’s guaranteed to be more specific than the one from the library.
CSS properties: background vs background-color – not sure if the latter isn’t still getting applied even if the CSS selector were more specific. In general I’d say that if you’re trying to override a style override it in the same way that it is written originally (hence: use background-color here), except if you’re absolutely sure that you know what you’re doing. I’ve done lots of CSS and couldn’t say without a test whether background would properly override an original background-color, because the latter is more specific and might take precedence? Would have to try it.
EDIT: And, as you say, your file load order should be fine. So the post title is probably a bit misleading, even though someone could learn a bit about reasoning about file load order when reading your post!