I must be missing something. I have tried to install several themes on the base Grav install (that comes with the Antimatter theme), and every time all I get is the Grav 404 page. ???
Worked through the theme tutorial and had no problems. There’s something I’m not getting about setting the site start page, I suspect.
Grav is not like a traditional CMS as you may be used to. There is not a fixed data structure, then a skin on top of that.
With Grav, there is a much tighter relationship between content and theme, in fact often the theme dictates what kind of data structure it supports. This allows Grav to be much more flexible than your typical CMS, but it comes at the cost of not being able to simple create your content, then flip through themes you like. You really have to pick the design that best fits your overall goal, then create or port your content to the structure that best fits your theme.
Each theme usually comes with a skeleton that provides example page structure, or sometimes demo content within the theme itself. Otherwise, it might come with some documentation on how to create content. Lastly, it might be built to work in place of the default Antimatter theme, and be a drop in replacement (like Afterburner2 and Bootstrap theme).
The best approach, is to download and install a Skeleton for the theme you have in mind if that’s possible, and look at the pages structure there.
Skeleton themes are great to learn from… but I too have suffered from 404 errors, but only on WAMP and only secondary pages (permissions issue? I dunno). Used XAMMP and it worked. Also, used mac (terminal - 'php -S localhost:8000) and it worked. Since Grav is supposed to be ‘super simple’ to get working, it might be worth looking into.
If you are getting 404s then its probably related to your PHP setup not having the appropriate bits enabled by default. See the docs here.
I’ have definitely seen this with some of the Windows WAMP stacks. Usually they come with mod_rewrite or mod_headers disabled.
For the record, Grav is very easy to work with. But for whatever reason, Windows continues to be a real pain to work with for any kind of web development. I strongly prefer working with Macs for this very reason, failing that Linux is getting pretty good 
I am covered with embarrassment. I know well just what Grav is, I’ve been active in development of a more conventional database-oriented CMS for 10 years and am intrigued by the difference. Although there are way too many eerie similarities, there are times I’ve wondered if this isn’t a file-based fork of MODX. I’ve detested Smarty for more than 10 years, and actually ran into Grav while studying and researching Twig, as MODX will most likely be migrating that way for its management interface in its next major version. And I’m working on MAMP. I installed the templates using the package manager. Of course, using a skeleton in the first place is no problem, but I need to figure out just what my problem is with an installed theme.
Ok, figured it out. Somehow I had gotten the idea that installing a theme would modify the user/config/system.yaml, but it doesn’t.
Actually I might add the option to set the theme in the system.yaml to the gpm install.
It’s not just the theme, it’s also the landing page. That was what was causing my problem.