Where can I get a mobile theme similar to the default Community?
Do you mean the mobile version of this site?
Vanilla asks people not to use the same theme styling as this site, which is why they don't make the theme available for download.
Vanilla ships with a default mobile theme. You can modify this.
Alternatively, there are a range of mobile themes in the addons section. If you add them to your site you should be able to select them from the dashboard.
Depending on what you want, you may need to adjust the theme using css.
My mobile users are very happy with the minusbaseline theme:
Most of the Mobile themes that I have tried. Does not give option to post .