Q: fill text box with background colour?

several ways to accomplish that.... aside manually stacking 'box'-generators under the title....

e.g. there are fonts with a background allready included, the famous 'BrakingBad'-style:

(search at dafont.com for 'braking bad' )

or…

Found this smart trick in one of Simon Ubsdell advices (coooomplex Motion projects …) look

https://youtu.be/VuBcAAJflX0?t=6m

The trick is:

create a clone of the text layer and apply a 'border-filter':

Starting with a 'title'project from Motions Templates

clone that layer…

in the Lib, goto Filters, Border, Simple Border, apply to clone

… and crank the Width up to 100 (or 200 .... just really, really thick)

finally, you have to re-arrange order, title on top/in front, done

now, that new title has the effect built-in, that the black box automatically resizes to text!

You can 'publish' tons of parameters (font, color of background etc), depending on what you finally need ...

A very handy trick, but Mr Ubsdell/Tokyo Productions  is well known for his Motion craftmanship ....:)

If you don't own Motion, leave here a message, I can welder you such a Title-generator within a few minutes, and publish it at the usual depository (fcp.co) ......

Very recently (maybe 2, 3 or 4 weeks ago), Alex Gollner as a guest of MacBreak Studio explained to Mark Spencer how to do exactly that.

I can't look it up right now, but you can easily google it. If you are not familiar with it, MacBreak Studio is a weekly video by Steve Martin and Mark Spencer of RippleTraining, always full of great advice about FCP X and Motion. I never miss it.