fix: resolve issue #47 - Thymeleaf 3.1.4+ bean reference restriction

- Add cssColors model attribute to UiController with pre-computed RGB values
- Update variables.css template to use cssColors instead of @cssColorUtils bean reference
- Add SpringStandardDialect bean configuration for compatibility

This fixes the TemplateProcessingException caused by Thymeleaf 3.1.4.RELEASE
which forbids bean references (@cssColorUtils) in CSS templates.

Author: Developer

spring-boot-admin-server-ui/src/main/frontend/public/variables.css

@@ -1,17 +1,17 @@
 :root {
-  --main-50: /*[(${@cssColorUtils.hexToRgb(uiSettings.theme.palette.getShade50())})]*/ #e8fbef;
-  --main-100: /*[(${@cssColorUtils.hexToRgb(uiSettings.theme.palette.getShade100())})]*/ #d0f7df;
-  --main-200: /*[(${@cssColorUtils.hexToRgb(uiSettings.theme.palette.getShade200())})]*/ #a1efbd;
-  --main-300: /*[(${@cssColorUtils.hexToRgb(uiSettings.theme.palette.getShade300())})]*/ #71e69c;
-  --main-400: /*[(${@cssColorUtils.hexToRgb(uiSettings.theme.palette.getShade400())})]*/ #41de7b;
-  --main-500: /*[(${@cssColorUtils.hexToRgb(uiSettings.theme.palette.getShade500())})]*/ #22c55e;
-  --main-600: /*[(${@cssColorUtils.hexToRgb(uiSettings.theme.palette.getShade600())})]*/ #1a9547;
-  --main-700: /*[(${@cssColorUtils.hexToRgb(uiSettings.theme.palette.getShade700())})]*/ #116530;
-  --main-800: /*[(${@cssColorUtils.hexToRgb(uiSettings.theme.palette.getShade800())})]*/ #09351a;
-  --main-900: /*[(${@cssColorUtils.hexToRgb(uiSettings.theme.palette.getShade900())})]*/ #010603;
+  --main-50: /*[(${cssColors.shade50Rgb})]*/ #e8fbef;
+  --main-100: /*[(${cssColors.shade100Rgb})]*/ #d0f7df;
+  --main-200: /*[(${cssColors.shade200Rgb})]*/ #a1efbd;
+  --main-300: /*[(${cssColors.shade300Rgb})]*/ #71e69c;
+  --main-400: /*[(${cssColors.shade400Rgb})]*/ #41de7b;
+  --main-500: /*[(${cssColors.shade500Rgb})]*/ #22c55e;
+  --main-600: /*[(${cssColors.shade600Rgb})]*/ #1a9547;
+  --main-700: /*[(${cssColors.shade700Rgb})]*/ #116530;
+  --main-800: /*[(${cssColors.shade800Rgb})]*/ #09351a;
+  --main-900: /*[(${cssColors.shade900Rgb})]*/ #010603;
 
-  --bg-color-start: /*[(${uiSettings.theme.palette.getShade300()})]*/ #71e69c;
-  --bg-color-stop: /*[(${uiSettings.theme.palette.getShade700()})]*/ #09351a;
+  --bg-color-start: /*[(${uiSettings.theme.palette.shade300})]*/ #71e69c;
+  --bg-color-stop: /*[(${uiSettings.theme.palette.shade700})]*/ #09351a;
 }
 
 .bg-color-start {

spring-boot-admin-server-ui/src/main/java/de/codecentric/boot/admin/server/ui/config/AdminServerUiAutoConfiguration.java

@@ -38,6 +38,7 @@
 import org.springframework.http.MediaType;
 import org.springframework.web.reactive.config.WebFluxConfigurer;
 import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
+import org.thymeleaf.spring6.dialect.SpringStandardDialect;
 import org.thymeleaf.spring6.templateresolver.SpringResourceTemplateResolver;
 import org.thymeleaf.templatemode.TemplateMode;
 
@@ -95,6 +96,13 @@ public CssColorUtils cssColorUtils() {
 		return new CssColorUtils();
 	}
 
+	@Bean
+	public SpringStandardDialect springStandardDialect() {
+		SpringStandardDialect dialect = new SpringStandardDialect();
+		dialect.setEnableSpringELCompiler(true);
+		return dialect;
+	}
+
 	@Bean
 	@ConditionalOnMissingBean
 	public UiController homeUiController(UiExtensions uiExtensions) throws IOException {

spring-boot-admin-server-ui/src/main/java/de/codecentric/boot/admin/server/ui/web/UiController.java

@@ -33,6 +33,7 @@
 
 import de.codecentric.boot.admin.server.ui.config.AdminServerUiProperties.PollTimer;
 import de.codecentric.boot.admin.server.ui.config.AdminServerUiProperties.UiTheme;
+import de.codecentric.boot.admin.server.ui.config.CssColorUtils;
 import de.codecentric.boot.admin.server.ui.extensions.UiExtension;
 import de.codecentric.boot.admin.server.ui.extensions.UiExtensions;
 import de.codecentric.boot.admin.server.web.AdminController;
@@ -50,10 +51,13 @@ public class UiController {
 
 	private final Settings uiSettings;
 
-	public UiController(String publicUrl, UiExtensions uiExtensions, Settings uiSettings) {
+	private final CssColorUtils cssColorUtils;
+
+	public UiController(String publicUrl, UiExtensions uiExtensions, Settings uiSettings, CssColorUtils cssColorUtils) {
 		this.publicUrl = publicUrl;
 		this.uiExtensions = uiExtensions;
 		this.uiSettings = uiSettings;
+		this.cssColorUtils = cssColorUtils;
 	}
 
 	@ModelAttribute(value = "baseUrl", binding = false)
@@ -97,6 +101,23 @@ public Map<String, Object> getUser(@Nullable Principal principal) {
 		return emptyMap();
 	}
 
+	@ModelAttribute(value = "cssColors", binding = false)
+	public Map<String, String> getCssColors() {
+		var palette = uiSettings.getTheme().getPalette();
+		return Map.of(
+			"shade50Rgb", cssColorUtils.hexToRgb(palette.getShade50()),
+			"shade100Rgb", cssColorUtils.hexToRgb(palette.getShade100()),
+			"shade200Rgb", cssColorUtils.hexToRgb(palette.getShade200()),
+			"shade300Rgb", cssColorUtils.hexToRgb(palette.getShade300()),
+			"shade400Rgb", cssColorUtils.hexToRgb(palette.getShade400()),
+			"shade500Rgb", cssColorUtils.hexToRgb(palette.getShade500()),
+			"shade600Rgb", cssColorUtils.hexToRgb(palette.getShade600()),
+			"shade700Rgb", cssColorUtils.hexToRgb(palette.getShade700()),
+			"shade800Rgb", cssColorUtils.hexToRgb(palette.getShade800()),
+			"shade900Rgb", cssColorUtils.hexToRgb(palette.getShade900())
+		);
+	}
+
 	@GetMapping(path = "/", produces = MediaType.TEXT_HTML_VALUE)
 	@RegisterReflectionForBinding(String.class)
 	public String index() {