feature: light/dark mode theme support (#208)

* feature: light/dark mode theme support

New feature providing a more modern flat design with support for light and dark modes, including:
- New SheepIt logo (old one removed)
- New configuration option (-theme) that changes the existing client theme.
- Minimal UI cosmetic changes (some separation lines under the logo and above the buttons)

src/com/sheepit/client/Configuration.java

@@ -64,6 +64,7 @@ public class Configuration {
 	private boolean autoSignIn;
 	private String UIType;
 	private String hostname;
+	private String theme;
 	
 	public Configuration(File cache_dir_, String login_, String password_) {
 		this.configFilePath = null;
@@ -89,6 +90,7 @@ public class Configuration {
 		this.extras = "";
 		this.autoSignIn = false;
 		this.UIType = null;
+		this.theme = null;
 	}