Refactored theme css, added Modern theme pre version

app/assets/images/logo_text_tr.png

app/assets/stylesheets/gitlab_bootstrap.scss

app/assets/stylesheets/main.scss

app/assets/stylesheets/themes/ui_basic.scss

app/assets/stylesheets/themes/ui_mars.scss

app/assets/stylesheets/themes/ui_modern.scss

app/helpers/application_helper.rb

app/views/profile/design.html.haml

app/views/refs/_tree_file.html.haml

lib/gitlab/theme.rb