Our CSS is kind of a mess in some places, so you might have to do some digging to find what you’re looking for. For those two that you’re looking for, the colours used for both are done using a sass variable, so you’d have to look for text like background: $primary-color;. The first rule is applied [in _buttons.scss](https://github.com/mattermost/mattermost-webapp/blob/master/sass/components/_buttons.scss#L28and the second is in _links.scss.
You may also run into the theme system as well if you’re trying to override the colours applied by that since all of that is done programatically, after CSS is applied.