To make a skin, you need to take a copy of resource/styles/steam.styles file and copy it to skins//resource/styles/steam.styes. The existance of that file will make Steam put that skin as an option in the settings->interface dialog (Steam will need to be restarted for it to show). From there you can start editing.
You can put new files or existing steam files you want to replace under your skins folder. Good luck!
== Tools ==
Hitting 'F6' key when running the Steam client toggles the VGUI editing dialog. You need to run Steam with the '-dev' command line parameter to have this work (running with -dev makes the Steam client run slower). When enabled, no other dialog can be interacted with; instead you can selected controls on any active dialog and get details on the selection.
There are several sets of information available about a selection. In the top-right is a link to the layout file which associated with the currently selected control, that you can click to edit.
The current layout script that is being applied to the control. This lists the script that applies to all controls in the current dialog or subpanel. You can edit the layout script with the link in the top-right of the dialog.
Has information about the currently selected control. Under 'Styles' it has the list of style keys that this control searches for in the styles files. Under 'Parents' is the list of parent control names that this child is under, which is also used to determine which styles should apply.
Shows all the current styles that are merged together to form the final style that is applied to a control, and from what file the style was loaded. The time item in the list is the merged version that is being