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)

build.gradle

@@ -32,6 +32,8 @@ dependencies {
 	compile 'net.java.dev.jna:jna:4.0.0'
 	compile 'javax.xml.bind:jaxb-api:2.3.1'
 	compile 'com.squareup.retrofit2:converter-simplexml:2.3.0'
+
+	implementation 'com.formdev:flatlaf:0.30'
 }
 
 jar {