Your goal could be realistic. If you're going to use 1 theming for multiple projects. Then you should create 1 theming. You can create this from an existing theme or build it completely yourself.
Editing an existing theme is always preferred, because creating a new one from scratch could be difficult. Especially when you're new to mendix and don't know the theming structure.
So for 2.5, you can download a default theme-package. Don't know if this one is also applicable for mendix v3. Then you could edit this one to make it look like your corporate website. Of course you could use a mendix v3 theming from the appstore to edit. That's it.
Maybe the Mendix guys should explain if this theme package is still applicable for v3 and if there are changes between the mendix v2.5 and v3.0 theming model.
I believe that the main theming problem lie arround the tab controler widget. If you figure out what the new layout is for the tab control i suggest you use firebug to detect the layout in a standard mendix 3.0 theme. You can also take a look at the default theming which is located in your deployment directory after you deployed your mendix 3.0 project without any theme selected.