Actually, the logo can be changed without creating a theme. However, other elements require a cusotm theme (where you want to edit speed macros). The logo can be found under "Control Panel"> "Portal"> "Portal Settings", and then click "Display Settings" on the right side.
In addition, you can also provide a small amount of css for the main topic, but at best this should be considered a temporary hack. Go to "Management"> "Page Layout"> "Open Pages", and at the bottom you will see a section with CSS that you can do:
#sign-in { display: none } .powered-by { display:none }
But keep in mind that a user with firebug or similar tools can still set the old input by changing CSS in firebug, and the Liferay tag line is still in the source, so it is hidden only from non-technical people. The topic is really the right decision.
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