This is a bare bones theme perfect for what you want to do. Have a peek

http://community.elgg.org/pg/plugins/project/790388/developer/sbarron/socialplus-theme-for-elgg-18

I'm not 100% sure about 1.8 as I haven't really looked into it. However for 1.7 you can do this by creating a plugin and copying all of the CSS from elgg/views/default/css.php into a new file and saving it as yourplugin/views/default/yourplugin/css.php

then in your start file call the following:

extend_view("css", "yourplugin/css");

Then you have a carbon copy of the default theme that is overriding the default theme. (Warning this doesn't include the contents of each plugin's css file). 

1.8 might be a similar setup, I'm not sure.

If you're a noob to elgg but consider yourself a dev in general, I would recommend looking at the elgg_development_tools plugin, which will create plugin skeletons for you. You can use them as a basis to start developing your own plugins to do what you want. This would get you started with playing around with the css you mentioned.

Just like any other dev (I think) we learn by doing(testing) far more than by reading. :)

@Trajan Thank you very much, that is exactly the type of info that I was looking for to get started !

Yes I think it's something missing to Elgg : a few dozens of pro themes arround 50$ each with some updates for browser compliance (there is probably a dozen arround but it isn't enough).

For example I'm looking for a Elgg theme that looks like the Buddypress theme below and I don't realy found any (I can't afford an original theme made by pro's otherwise there would be no problem ;-)

http://themetastic.com/buddypress/?wptheme=Studio

I am still looking for a decent theme with no luck.  Have not found a single PRO Elgg theme compatible with 1.8

The 1.8 official release is still very new, and many people are still working within 1.7 for existing projects.  1.8 stuff will come in time, but unless you hire someone to build a custom theme you have to wait for people to develop at the rate determined by their free time.

Its only 2days since Elgg 1.8 is out. So there are not much plugins for 1.8yet. You can try tidypics plugin which is the official photogallery plugin for Elgg.

Ohh ok, no problem then.  I did not realize it's only been few days, my apologies :)

It usually takes most devs at least a week or two to update their plugins, so I will just play with what I have.

Thank you very much for the reply !  First time I post a question that was this clueless and don't get flamed.  Great to see such a helpful community :)

there is plugin called deyan shell by angel gabriel that deals with 1.8 theming.. check out ;)