As of January 2016, the Caltech Overleaf portal offers thesis authors a LaTeX template that reflects the regulations of the Office of Graduate Studies and the requirements of the Institute. There are two versions of the thesis template available: one with the approved Caltech logo; the other without a logo.
You can download the Overleaf template files as zip files to your own computer if you prefer to use them that way. Open up a blank thesis template on the Overleaf site, then click on Project at the top of the screen. A green box labeled “Download as ZIP” will show up in the left column, and you can choose which types of files you want to download.