You have one or both of the following problems:
a) You open a webscreen in Chrome (or other browser) and it does not render properly. The screen seems to be missing text, and text appears in the incorrect locations. 
b) You get errors in console with a message indicating "Failed to load resource: the server responded with a status of 404 (Not Found)". 
Example (in Chrome):

c) Your OutSystems Platform is running on Windows 2003 (i.e. using IIS6).

The new London theme uses embedded fonts (in .woff format). This format uses a mime type that is not registered by default in IIS6 (the one used in Windows Server 2003). As such, IIS 6 will not be able to serve requests for files with that extension, throwing a "404 Not Found" error, as seen in the previous image.
Add the mime type to IIS6.
1) In IIS Manager, right click the local computer and select "Properties"
2) Click on the "MIME Types..." button
3) You'll see a list of extensions and mime types. You can check that .woff extension is not present in the list.
To add it, click on the "New..." button
4) Add the following values:
Extension: .woff
MIME Type: application/font-woff
5) Press "OK" buttons of all opened windows.

6) Restart IIS so that the change takes effect.

And you're done!

The web page should now render properly.

With best regards,
Mário Pires