You can controll it in the theme roller propety under navigation
I guess my question was addressed to the Oracle APEX team...what was the rationale behind the decision to change the default look-and-feel by introducing such a (IMHO jarring) contrast between the header and sidebar background colors and thereby force all customer applications that use the out-of-the-box default UT theme/style to make custom changes to preserve APEX 5.1 UI. Why not introduce this as a template option that users could voluntarily choose? Hmm since APEX 18.1 is not GA yet, this may very well be how it works, in that case, my apologies :-)
It is voluntary, you don't need to verify your theme.
So your existing applications may retain their existing skin, or you can choose to move forward with an upgraded UT full of tweaks, adjustments, improvements, and perhaps mild colour changes while no doubt seeking the perfect contrast.
There was a similar difference between 5.0 and 5.1.
Thanks for bringing this to our attention - it was indeed an oversight that the page alias hadn't been defined. I've just updated the app, and the link should work without issue now.