how can i change look of windows

As far as I know, there is no (easy) solution to change the look of any window (e.g. window decorators), because i3 is window manager and NOT a desktop environment. What you have, nonetheless, is full control over the colors used for the windows. I adapted a zenburn style and am very happy with it.

Screenshot
i3-config


# color defines for zenburn styled i3
set $bg     #2c2c2e
set $fg     #9f9f9f
set $hi     #efef8f
set $ac     #a0afa0
set $tx     #040404
set $ia     #8f8f8f
set $be     #8faf9f
set $yw     #ccdc90
set $gn     #88b090
set $rd     #e89393

# set some nice colors      border     background  text
client.focused              $ac        $ac         $tx
client.unfocused            $bg        $bg         $ia
client.focused_inactive     $bg        $bg         $ac
client.urgent               $rd        $rd         $tx

# Start i3bar to display a workspace bar (plus the system information i3status
# finds out, if available)
bar {
    status_command i3status --config ~/.i3/i3status.conf

    colors {
        background          $bg
        statusline          $fg
        separator           $hi
        focused_workspace   $gn        $bg        $ac
        active_workspace    $gn        $ac        $tx
        inactive_workspace  $bg        $bg        $ia
        urgent_workspace    $rd        $bg        $ac
    }
}