iMovie offers only six themes! how can I install more??

You can't.
Make your own!

A Theme is a designed, repeated (=structure!) element in your VidCast.

A nice Title-chart is a Theme.

A collage of stills is a Theme (here, some stone-old 'Still comes alive!' effect from my website CLICK)

... .any very short 'movie' is a Theme - do you remember the TV-Show from the 80ies, showing the house of our heroes? or some silly 'animation'? 
if your business is cats, show 60 cats in 20secs.
if your business is Jazz from the 30ies - Ken Burns some níce record-covers, use the 'effects' to make it more 'themey'
if your business is your son's soccer-career, show some hillarious fans and 'GOAL!!' as soundtrack

produce this 'Theme-clips' as an iMovie-project. choose 'Share/Export using Quicktime', choose again Quicktime, Options, Codec: AppleIntermediate
=> this results in a 'lossless', high quality file, you can copy and insert again and again. 
Create an Event with your shows elements, to have instant access to all repetitive elements of your show.

Ask a friend to design you a logo .. 
This is done with a 5y-old, 89$ 3D-app, still existing on one of my backup-drives (Carrara):

Some moving picture(s), some audio-'signal' (=the one in my demo is from iM`s Library, 'Electro Static Accent' - fits nicely to a bla-bla-blog) and off you go with a Theme.

In Radio business, they like to call that a 'station identifier' - the Theme should identify YOUR  VidCast - don't use a ready-made, make your own!