First create a new theme in the xpage applicaiton.
In Lotus Notes 8.5.2 designer go to resources -> Themes
Right click on Themes and click New Theme...
A new theme will come up. Call it oneui or blue
Find the code that says <theme extends="webstandard">
and change it to <theme extends="oneui">
Save the theme and close.
Now double click on "Application Properties" (last entry below Resources and Composite Applications)
click on the XPages tab at the the bottom of the top section that opens.
Select the Application theme and set it to the name you chose earlier.
NB You can have different themes for Notes Client and web if you like.