Create .mintty folder with subfolder themes in c:\Users\<username>\.mintty\themes
Example:
C:\Users\sandeep\.mintty\themes
Download '.minttyrc' files from https://github.com/iamthad/base16-mintty and add them to themes folder.
or download directly via DownGit -> the createdLink
For me the proper location with Cygwin was: C:\cygwin64\home\atem18\.mintty
Open git bash for windows --> options --> theme dropdown