(I'm on support staff at Overleaf.) As mentioned in the comments, we're still running TeX Live 2017 for new projects. We know that we need to stay more current and we're working on releasing an updated TeX Live distribution for new projects in the coming months.
Since the package in question was added to CTAN in 2018, it's not yet available on Overleaf at the time of writing. Generally, for relatively simple, self-contained packages, you can download the .sty file from CTAN and upload it to your Overleaf project to use it. As long as the package code does not depend on new features of other packages or changes to binaries distributed as part of a newer TeX Live version, it's possible to use updated or new packages on Overleaf this way.
However, since this package was distributed to CTAN as a .dtx rather than a .sty, the simplest option is to download the .sty file from the TeX Live sources for the package. This .sty file can then be uploaded to your Overleaf project without needing to process the .dtx file separately.
=is nice on a blackboard, but poor in something typeset. Write it in words instead. – daleif Aug 07 '19 at 15:16.sty, but again, I would not use this. – daleif Aug 07 '19 at 15:34