That's how Themes work. Please read this article to understand further about Themes:

http://support.mozilla.com/en-US/kb/Using+themes+with+Firefox

Add extra empty toolbars if you want to see more of the persona.
You can create new toolbars in View > Toolbars > Customize
You have to place something on a toolbar (drag a Space on it) before closing the Customize window as empty toolbars are automatically removed.

See http://kb.mozillazine.org/Toolbar_customization

Another possibility: You can create one toolbar (name: PersonaFiller) via "View > Toolbars > Customize" and put a single Space item on it and use code in userChrome.css to set a min-height for that toolbar.
If you do not want to see that toolbar to get more website space then you only need to remove one check-mark via the right-click context menu

#__customToolbar_PersonaFiller {min-height:100px !important;}

(adjust the height to suit your needs)

Add code to userChrome.css below the @namespace line.

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

See http://kb.mozillazine.org/Editing_configuration#How_to_edit_configuration_files

If you want to hide the Tab bar then see:

Tools > Options > Tabs: [ ] "Always show the tab bar"

You can add code to userChrome.css below the @namespace line.
See http://kb.mozillazine.org/Editing_configuration#How_to_edit_configuration_files

/* Show Close Button On Last Tab */
.tabbrowser-tabs[closebuttons="alltabs"] > .tabbrowser-tab > .tab-close-button {
 display: -moz-box !important;
}
.tabbrowser-tabs:not([closebuttons="noclose"]):not([closebuttons="closeatend"]) > .tabbrowser-tab[selected="true"] > .tab-close-button {
 display: -moz-box !important;
}

Set the pref browser.tabs.closeWindowWithLastTab to false on the about:config page to prevent closing the last tab from closing the browser.

You can also look at this pref: http://kb.mozillazine.org/browser.tabs.closeButtons

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.