Table not getting created upon activation of WP Theme

dbDelta is extremely picky. Looking at your code I’d say you’ve violated this rule:

You must put each field on its own line in your SQL statement

Two of your fields are sharing lines with parenthesis. I haven’t tested that particular pattern but dbDelta is extremely picky.

I don’t know if that space after NOT NULL will matter.

I doubt that checking for the existence of the table is necessary. Checking your option should be sufficient.

And finally, register_activation_hook is for plugins, not themes. There is no similar specialized theme activation hook that I am aware of. There are a number of work-arounds that have been suggested if you search this site for them.