I use
UltraModernUI for NSIS, which leverages the
SkinnedControls plugin, and made a modern flat look UI, with custom buttons and scrollbars etc. But it was quite a bit of work to skin almost everything (message boxes still aren't skinned).
There's also
Graphical Installer,
and
SkinCrafter (the latter has issues with 2.46+, apparently).
None are perfect. If you use plugins to generate any content, you'll need to use SetCtlColors.
I'd show you a screenshot, but it's made for a third party.