Hi Katerina,
This all depends on if you are in Editing Mode, which you are in, by the look of your screenshots, as I can see the editing icons in the blocks. This happens in some pages because they have been designed that way. It's not a fault it's just the way Moodle works.
When you turn editing ON you see ADD BLOCK in either the left side bar or the right, depending again on the page you happen to be in. Postit has the following settings which you can alter if you want to.
These are found in ../theme/aardvark_postit/config.php
$THEME->layouts = array(
'base' => array(
'file' => 'general.php',
'regions' => array('side-pre', 'side-post', 'pbar-pre','pbar-post'),
'defaultregion' => 'side-post',
),
'general' => array(
'file' => 'general.php',
'regions' => array('side-pre', 'side-post', 'pbar-pre','pbar-post'),
'defaultregion' => 'side-post',
),
'course' => array(
'file' => 'general.php',
'regions' => array('side-pre', 'side-post', 'pbar-pre','pbar-post'),
'defaultregion' => 'side-pre',
'options' => array('langmenu'=>true),
),
'coursecategory' => array(
'file' => 'general.php',
'regions' => array('side-pre', 'side-post', 'pbar-pre','pbar-post'),
'defaultregion' => 'side-post',
),
'incourse' => array(
'file' => 'general.php',
'regions' => array('side-pre', 'side-post', 'pbar-pre','pbar-post'),
'defaultregion' => 'side-pre',
),
'frontpage' => array(
'file' => 'general.php',
'regions' => array('side-pre', 'side-post', 'pbar-pre','pbar-post'),
'defaultregion' => 'side-post',
'options' => array('langmenu'=>true),
),
'admin' => array(
'file' => 'general.php',
'regions' => array('side-pre', 'pbar-pre','pbar-post'),
'defaultregion' => 'side-pre',
),
'mydashboard' => array(
'file' => 'general.php',
'regions' => array('side-pre', 'side-post', 'pbar-pre','pbar-post'),
'defaultregion' => 'side-post',
'options' => array('langmenu'=>true),
),
'mypublic' => array(
'file' => 'general.php',
'regions' => array('side-pre', 'side-post', 'pbar-pre','pbar-post'),
'defaultregion' => 'side-post',
),
By changing 'side-post' to 'side-pre' where I have highlighted them in yellow, you can eliminate that partucular problem occuring again.
Like I said it's nothing to do with the theme it's a Moodle thing as other themes do the same, when in editing mode. Turn editing OFF and it all goes back into place...or should if you have the updated version of Moodle for whatever version (Moodle 2.0.x; 2.1.x or 2.2.x)
HTH
Mary