They're for decoration only.

the picture shows him accessing his horses inventory

the book says to sneak and access the sattle bags ... but in the book it lists which horses have what specs i do believe only a few horses actually have the saddle bags as i have the same issue and cannot access them. like i said i think it depends on which horse your riding ... i do know that the dwarven horse in markarth allowed me to use his saddle bags but thats the only one that worked ... glitch maybe?? idk

you need skse. Then you open up the main menu in game (esc) go to Mod Configuration Menu (usually referred to as simply "MCM"), which is on your left hand panel beneath options, I think, but a little past mid way down. You want to click on that. A new menu will open up- the MCM (Mod Configuration Menu). Here you want to select, on the left hand side (it looks very much like the escape menu) the mod you want to change. It will be listed. When you've opened up Convenient Horses, you might have to do a little digging of your own past this point, I think it's an option on the first tab, but you can figure it out. You're looking for the option for the key to open your horse's inventory. Click on it, and then press the key you want to use. Congrats! You have now made it so it will be easy from hereon out to access your horse's inventory. There are some bugs now (my horse inventory keeps getting deleted, and there's a long pause before the horse inventory opens) but maybe I'll have better luck in the future. GL/HF.

You forgot Arvak. Maybe a translucent bluish color would do the trick!