I don't think you can lock the blocks so that they can't be moved to the dock - they are designed so that a user can dock or show them as they wish. There have been some discussions about forcing the blocks starting position (docked/undocked) check out the links below
Also I think the colour option in splash is there in the theme and the only way to stop users from choosing a different colour would be to remove that option from the display (possibly a conditional statement allowing it for users you want to leave it with but hiding it from others?)
As for the page background - it depends if you mean the background of the content itself, or of the wider whole page.
The background of the whole page is part of the image that gives the coloured bands at the top of the page - it is actually a 6000px high narrow image repeated across the screen, so to change that you would have to alter the image itself. For the content, firebug shows the rule below. You could locate that rule (it is in sl.css - line 354) and either change it there (or rather if you are making changes to a core theme, in your cloned copy of splash!) or you could add that rule and modify it in the custom css part of the splash theme settings page
background: none repeat scroll 0 0 #FFFFFF;
Eliminating the right hand column permanently is much more difficult to do as that would alter the main layout of the page and may need some recalculating of the layout css. However, you may be able to work around that by editing the available regions for each page layout type in the config.php so that they layouts do not call for side-post (right hand column) in most cases e.g.
'standard' => array(
'file' => 'general.php',
'regions' => array('side-pre', 'side-post'),
'defaultregion' => 'side-postpre'
Or, you could edit the blocks themselves to all appear on the left in each block's settings.
Not having any content in the right hand column (side-post) should then allow the middle column to fill the space. However, you may find that the page layout css could need some work if this is to be the case permanently for your site.
as a starting point at least