Windows, fields, portals and web viewers can all have scroll bars. For which object(s) are you trying to set this? I don't think that you can specify different colors for different parts of the scroll bar, but you should be able to change it's color as a whole by changing a border color.
I actually have this question, too. I have been unable to find any way to modify the color of the scroll bars contained within a portal. I also cannot figure out how to modify the arrow that's used on a drop-down menu or pop-up list. I have spent quite a bit of time trying to search for answers online. The closest I have found is a thread talking about modifying the css/xml file that contains the hex color code styling within the FM theme itself. I duplicated a pre-existing theme, saved over it, and then changed the styles for my database. My only experience with FM are versions 12 & 13 so I'm not sure if the css/xml hint I stumbled upon is even applicable to FMPA. I would like to know if there is a "right" way to go about this because the FileMaker Training Series book I have has very little styling information in it. Modifying the border color only modifies the rectangular boundary of the portal itself. There is no way to select the scroll bar itself. Any ideas? Thank you in advance!
When I started to read your answer, modifying the actual theme file was the only option that came to mind. That's a risky endeavor and not one I can recommend as a mistake can cause a lot of issues and you may find that you have to do it all over again with the next release of FileMaker (or after any re-install of the application).
I have been able to get a bit sneaky when it comes to putting an arrow of the type I want on a drop down list. I specify NO arrow in the Inspector and then drop a graphic image of the arrow I want on top of the field. In my case, the drop down list was hidden behind another field and I wanted that field to look like a drop down list even though it wasn't actually a drop down list to encourage users to click on the field--which then deployed the hidden drop down list.