You could, however then you need to set up an array to track which screens have which logo, so when a screen closes you can change the logos back to what they were prior, and whenever the logos are changed, it flickers the default blue theme until it can display the logo you want. To avoid that I suppose you could use a single roImageCanvas for all 'screens' and manually program the logo graphics, positioning, background, and item displays. Or just ignore the changeover sequence.
Maybe some day Roku will have the capability of having more than a single user-defined theme. You can use the overhang secondary logo wherever you like. Just if one is set, it'll be displayed. So if you define the primary logo AND the secondary logo, both will be drawn on the screen. If you just have the secondary defined, then the secondary will be displayed - if you follow.
What we've done before is monitor the close event for the screen on top, when the screen is closed it refers to the array and subtracts 1 from the array index and resets the theme with the saved logo name. When a new screen opens we grab the new logo name from the xml, push the name into the array (to increase it by one), and reset the theme again. There might be a better way, but that worked for our purposes.