Hi Wojtek! Looks good, sure different to every other theme! But I have a couple points..
from a design view.....
* On the blog module the widgets are repeated twice, once on the side and once at the top.
* same with the ecommerce module, the shopping cart is at the top of the right hand column, It would work better on the left under the menu.
* staying with the ecommerce page, its got a bit of blue and white from the old blackcandy styles. It might look better if you change it to something more alike to your theme
* forum page - I would make the currently online font smaller and change that so it doesn't take up as much room as it does now and get rid of the white bg
onto the coding side a couple points, It looks really good (one of the tidiest themes ive seen) but ......
* SilverStripe has html conventions. http://doc.silverstripe.com/doku.php?id=html - basically #IDs are UpperCamelCase and class's are lowerCamelCase. Its not a major issue but just a note if you do any more themes that ids should be Upper.
apart from that the CSS looks fairly tidy and its indented and commented. Just watch out in you keep your indenting correct, you have over indenting a few things. nothing major just looks better and more professional and makes it easier to read for people who use it
Good work on the theme! Once you have submitted it to silverstripe.com/themes and it gets approved I will close your task!