Add highlight matcher

src/main/java/org/zwobble/mammoth/internal/styles/HighlightMatcher.java

@@ -0,0 +1,16 @@
+package org.zwobble.mammoth.internal.styles;
+
+import java.util.Optional;
+
+public class HighlightMatcher implements DocumentElementMatcher<String> {
+    private final Optional<String> color;
+
+    public HighlightMatcher(Optional<String> color) {
+        this.color = color;
+    }
+
+    @Override
+    public boolean matches(String color) {
+        return !this.color.isPresent() || this.color.get().equals(color);
+    }
+}

src/test/java/org/zwobble/mammoth/tests/styles/HighlightMatcherTests.java

@@ -0,0 +1,38 @@
+package org.zwobble.mammoth.tests.styles;
+
+import org.junit.jupiter.api.Test;
+import org.zwobble.mammoth.internal.styles.HighlightMatcher;
+
+import java.util.Optional;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.equalTo;
+
+public class HighlightMatcherTests {
+    @Test
+    public void highlightMatcherWithoutColorMatchesAllHighlightElements() {
+        HighlightMatcher matcher = new HighlightMatcher(Optional.empty());
+
+        boolean result = matcher.matches("yellow");
+
+        assertThat(result, equalTo(true));
+    }
+
+    @Test
+    public void highlightMatcherWithColorMatchesHighlightWithThatColor() {
+        HighlightMatcher matcher = new HighlightMatcher(Optional.of("yellow"));
+
+        boolean result = matcher.matches("yellow");
+
+        assertThat(result, equalTo(true));
+    }
+
+    @Test
+    public void highlightMatcherWithColorDoesNotMatchHighlightsWithOtherColors() {
+        HighlightMatcher matcher = new HighlightMatcher(Optional.of("yellow"));
+
+        boolean result = matcher.matches("red");
+
+        assertThat(result, equalTo(false));
+    }
+}