Fix read/write tab colors

[xtine]

Make sure Read and Write tab colors are correct, per #256

When hovering over Write tab:

[donjo]

I think maybe we should do a lighten(10%, @grey) (or whatever the variable is for the navbar) for the background color when hovering over the non-active tab. Taking white out makes sense but I think it should have a little bit of a button effect.

[donjo]

Another thing that changed at some point (which is probably what made that link blue in the first place) is causing the icon of the active tab to be too close to the text. The text for the active tab should get a padding: 0 5px which used to be applied by a .preamble-header span rule that still applies to the inactive tab but not the active one.

[xtine]

So there was a few padding changes due to the fact that mobile hides the "the proposal" and "a comment" text. I've added left padding to the active mode link to account for that.

[xtine]

@donjo: I added the lighten gray hover on the non-active tab and the hover gives a nice pop effect. Good suggestion:

[donjo]

Looks good 👍

[cmc333333]

Two thumbs!