No tutorials that I know of, but here's a quick and dirty one to get you started:
After you install UMUI, make a copy of one of the skin .nsh files in C:\Program Files (x86)\NSIS\Contrib\UltraModernUI\Skins and copy the corresponding folder of images, and move them to your installer folder. Grab Paint.NET or Gimp and change the buttons and scrollbars to a flat color, and use a slightly different color as a border. Take a screenshot of one you want to duplicate and use the eyedropper tool to get the exact color if you want. Don't bother making the background/header/bottom, etc. a single color; you can just disable those. If you use the components page, you probably want to make new checkboxes too. Copy and edit one of the ones from C:\Program Files (x86)\NSIS\Contrib\Graphics\Checks, and specify the new image with MUI_COMPONENTSPAGE_CHECKBITMAP.
!import the skin's .nsh file into your installer, and tweak it to remove the images you don't want and to change the paths to your new versions, tweak the colors as necessary. I'd suggest specifying the following in the .nsh:
!define MUI_COMPONENTSPAGE_SMALLDESC
!define UMUI_NOLEFTIMAGE
!define MUI_ABORTWARNING
!define MUI_UNABORTWARNING
!define UMUI_USE_ALTERNATE_PAGE
!define UMUI_USE_UNALTERNATE_PAGE
Note that some defines start with UMUI_ for UltraModern as they're UMUI only, others with MUI_ as they're shared with Modern UI 2.
Check out the examples in C:\Program Files (x86)\NSIS\Examples\UltraModernUI (there are some new pages you can play with for a more modern feel, though they mostly duplicate InstallShield behavior).
The readme is great, but very dense as there are tons of options (hit Expand All at the top if you want to do searches): C:\Program Files (x86)\NSIS\Docs\UltraModernUI\Readme.html
