Random Selection Percentages?

[Randomblink]

Ok...
Here is my conundrum...

I am creating a db that lets Role-Playing Gamers put together an adventure with a few clicks of the mouse and a printout versus the hours that it normally takes. So, with my explanation upfront here goes...

I have a table...

[tbl_Themes]
<ThemeID> - Autonumber - Primary Key
<ThemeTitle> - Text
<ThemeDescr> - Memo

What I want to do is the following:

Create a form where as the user is filling in the various parts of an adventure, that when they get to deciding on the theme they have several options:

Option 1)
Make up a theme, and type it into the <ThemeTitle> field and go on...

Option 2)
I have several themes already listed, I want the user to press a button and have it randombly select one of my pre-set themes and then they can move on...

Option 3)
Here is where it gets tough. I want the user to be able to add more themes to the list, and then go back and click on the random-selection button and have it randomly select from one of the 7 originally in there including any the user has added.

The 7 already in there are:
Action/Adventure
Comedy
Espionage
Horro
Mystery
Revenge
Romance

So if the user adds in Bounty Hunt I want the random-selection to include Bounty Hunt as an ADDITIONAL selection that COULD be selected by randomly selecting from the list...

First off: I am fuzzy about having the computer randomly select anything.

Secondly: I have NO CLUE how to tell the computer to randomly select from a range of options that can grow. I have an idea, but no code to back them up...

First-Tell the computer to count all of the selections possible (code?)
Second-Have the computer assign a Number to each? (code?)
Third-Randomly select a number from 1 to <last number of option in list>

If you wanna help, I would appreciate it. I am working on a gaming item because I learn best when working with fun databases and I am ready to learn more...

HELP!