This is basically a default style with a few modifications that make it look more modern (IMO):
1) Removal of some gradients for flat appearance
2) Removal of curves
3) Slight tweaks to colors
4) Main background color is very light gray rather than white
5) Thread lists have alternating rows of very light gray/light gray backgrounds for improved readability (IMO)
Installation instructions:
1) Backup current contents of /styles/default/xenforo/gradients (will be overwritten)
2) Upload contents of Upload directory to forum root
3) Import style-Default-Style.xml
4) Copy contents of ExtraCSS.txt to EXTRA.css
