You can use themes listed below to modify WinRAR icons. Just download a theme archive and open it in WinRAR to install. Installed themes can be selected in the "Options/Themes" WinRAR menu. If you wish to create a new theme, please read the following document.

WinRAR Smooth Green theme, designed by Strek

Download:
  version with 48x48 toolbar icons

WinRAR techfacts.net theme, www.techfacts.net

Download:
  version with 48x48 toolbar icons

WinRAR Cryo64 theme version 1.01 © by Cryo64, www.cryo64.com

Download:
  version with 64x64 toolbar icons

WinRAR CodeHammer v4i theme by Wael El-Ghazzawi (CodeMasterVII)

Download:
  version with 32x32 toolbar icons

WinRAR Designcontest theme by Bohdan Zograf, www.designcontest.com

Download:
  version with 32x32 toolbar icons