feat: support multiple color themes

[unhappychoice]

Summary

Add support for multiple color themes to customize the output appearance.

Proposed Themes

• steam (default) - Steam blue gradient
• valve - Valve red/orange
• portal - Portal blue/orange
• dota - Dota 2 red
• csgo - CS:GO yellow/orange
• custom - User-defined colors

Interface

steamfetch --theme valve
steamfetch --theme custom --primary "#FF0000" --secondary "#00FF00"

Implementation

• Add --theme CLI option
• Define color palettes for each theme
• Apply theme colors to ASCII art and stats