Go to the "Interface Styles" plugin on Runelite and select the 2005/2006 Gameframe.
Add "Resource packs" plugin on Runelite from the Plugin Hub section.
Download the file here on github as a zip, (Click the Green button where it says code) and extract it.
Paste the path of the 2005/2006 folder into "Resource packs" settings like so:
If any of my changes are not to your liking, simply go into the 2005/2006 folder and delete whichever files you didn't like and it'll revert back to Runelite's version. Then just turn Resource packs plugin off and on again.