We created a provider hosted webapp in sharepoint 2013. The webapp is accessed thru an iframe. We use twitter bootstrap for the UI in the webapp. Does anyone know of a bootstrap theme that makes the app look like standard sharepoint 2013 UI?
Assuming you mean that the colors should match the colors of the current SharePoint theme, you can use this approach - How-to-Easily-Bring-SharePoint-2013-Theme-Colors-Into-Your-Apps
When you have a IFrame only app, you can use this to inherit the CSS from the parent - https://stackoverflow.com/a/4613196 and apply some SP classes to your markup to get the colors.
If you just want to have the colors that are definded in the standard Composed Look "Office" you can easily create your own theme based on the corresponding colors and fonts - Themebuilder
If you want a more complex solution you have to point out what exactly you want as result.