Add README

Author: the1812

README.md

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+# SteamCSS
+Insert custom CSS to Steam Library and Browser.
+
+## Usage
+Download `SteamCSS.zip` at [Releases](https://github.com/the1812/SteamCSS/releases) page.
+
+There are 3 files:
+- `SteamCSS.exe`: Main program
+- `SteamPath.txt`: Specify Steam install path
+- `Style.css`: Your custom CSS code to insert
+
+Write your Steam install path in `SteamPath.txt` and custom CSS code in `Style.css`.
+
+Start `SteamCSS.exe`, it will insert CSS code into Steam files and exit silently.
+
+## Notes
+- The default `font-family` is from [Sarasa-Gothic](https://github.com/be5invis/Sarasa-Gothic).
+- This only applies on library and built-in browser, you may need [MacType](https://github.com/snowie2000/mactype) to change Steam UI font.
+- Steam files will be overwritten after some Steam updates, and you have to rerun this program to insert CSS again.