Formatting Question for Wordpress Plugin

Good morning!

Just recently discovered H5P and am totally impressed.  I run a music education site for teachers and plan to use many of the examples there.

I'm using the wordpress plugin and the image slider and I'm trying to figure out how to format something. 

Some of the images are different sizes so I get a black background that fills in the space

Example:  https://screencast.com/t/5womwjOSRnTx

I'd like to change that background to white, it would completely solve my issue since I'm just showing music.  I checked the console in Chrome to find out what stylesheet to change and I found the line of code, but it's in a folder I can't seem to access called "cached assets".

-- >  wp-content/uploads/h5p/cachedassets/

I can get to the h5p folder, but there's no cached assets folder.  Is it hidden?  Any suggestions?

FYI,I'm not developer, I just know enough to be dangerous.  :)

Thanks for any advice.