Use mozilla colors (fixes #189)

I tried to fix #189 and this is what I came up with.

I am running icedoce 24.8.1 on debian wheezy with conversations 2.7. So this is
the environment in which I created and tested this code. I then ported it to
master on the git repo. Also I am not using the full feature set of this
plugin. So this patch should really be tested by someone before merging.

I basically replaced hard coded colors by values as documented in the
[mozilla developers docs](https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Mozilla_Extensions#Xref_csscolorvalue()).
I tested it with some GTK themes and for me it works fine.

There are however some kinds of colors I could not find a variable for. In
those cases I looked at what the core thunderbird content does and this is
what I came up with:

-   `threedshadow` is used for borders. This does work fine in with most themes
    but can be a bit too low-key in some dark themes.

-   `rgba(0,0,0,0.25)` is used for shadows.

-   `#888a85` is used for secondary text.

-   `black` and `white` are used for textarea color/background.

Note that I simplified some things, i.e. there were more different colors
before this patch. In many cases I was not sure if the diversity in shades of
grey was intentional or not. As I was already struggeling with the available
color variables I decided to go with less diversity.