This is the best method I've found for ultimate flexibility on color and position for YouTube captions. You can individually change the color of the border and shadow of the text. You can also change fonts, sizes, etc.
There's a small price to pay, in that it's a two step process, but the second step is just drag and drop.
Download Aegisub from
https://aegisub.en.softonic.com/
Download YTSubConverter from
https://github.com/arcusmaximus/YTSubConverter
All the caption files for my template video are available for download here
http://ittextra.com/techPop.jsp?id=93. You can download them and test them on your own videos.