Improve settings

This PR is in anticipation of these changes https://github.com/atom/settings-view/pull/277 in the settings. __tl;dr:__ The settings will use the `ui-variables` and match a theme's look and feel.

There isn't much to do for themes, but this PR still adds a couple improvements. Like:

## Before

![screen shot 2014-12-18 at 9 53 31 am](https://cloud.githubusercontent.com/assets/378023/5483250/340d3772-86b2-11e4-9fe4-c237362effdb.png)

![screen shot 2014-12-18 at 11 05 47 am](https://cloud.githubusercontent.com/assets/378023/5483255/435067ea-86b2-11e4-8b27-4aae2fd094fc.png)

## After

![screen shot 2014-12-18 at 10 43 16 am](https://cloud.githubusercontent.com/assets/378023/5483253/3c731e40-86b2-11e4-9524-1658997751b9.png)

![screen shot 2014-12-18 at 11 06 00 am](https://cloud.githubusercontent.com/assets/378023/5483257/4ec55b76-86b2-11e4-8c72-10f0a0a1d70c.png)

ps. This part https://github.com/simurai/seti-ui/blob/fe45bccf324a5b02b370bb5cfb4b51bc6245b081/stylesheets/settings.less#L9-L16 might not be needed if the overal `@text-color-subtle` would be a bit lighter.