diff --git a/klass-forvaltning/pom.xml b/klass-forvaltning/pom.xml
index 1664be10..1e224ea9 100644
--- a/klass-forvaltning/pom.xml
+++ b/klass-forvaltning/pom.xml
@@ -9,6 +9,9 @@
 
     <properties>
         <maven.deploy.skip>false</maven.deploy.skip>
+        <java.version>8</java.version>
+        <maven.compiler.source>1.8</maven.compiler.source>
+        <maven.compiler.target>1.8</maven.compiler.target>
     </properties>
 
     <parent>
diff --git a/klass-forvaltning/src/main/webapp/VAADIN/themes/klasstheme/styles.css b/klass-forvaltning/src/main/webapp/VAADIN/themes/klasstheme/styles.css
index f4700975..9a211de5 100644
--- a/klass-forvaltning/src/main/webapp/VAADIN/themes/klasstheme/styles.css
+++ b/klass-forvaltning/src/main/webapp/VAADIN/themes/klasstheme/styles.css
@@ -3070,7 +3070,7 @@ div.v-layout.v-horizontal.v-widget {
 	text-align: center;
 	top: 50%;
 	line-height: 1;
-	margin-top: −0.47em;
+	margin-top: -0.47em;
 }
 
 .klasstheme .v-filterselect [class$="button"]:hover:before {
@@ -3547,7 +3547,7 @@ div.v-layout.v-horizontal.v-widget {
 	text-align: center;
 	top: 50%;
 	line-height: 1;
-	margin-top: −0.47em;
+	margin-top: -0.47em;
 }
 
 .klasstheme .v-filterselect-tiny [class$="button"]:active:after {
@@ -3674,7 +3674,7 @@ div.v-layout.v-horizontal.v-widget {
 	text-align: center;
 	top: 50%;
 	line-height: 1;
-	margin-top: −0.47em;
+	margin-top: -0.47em;
 }
 
 .klasstheme .v-filterselect-compact [class$="button"]:active:after, .klasstheme .v-filterselect-small [class$="button"]:active:after {
@@ -3806,7 +3806,7 @@ div.v-layout.v-horizontal.v-widget {
 	text-align: center;
 	top: 50%;
 	line-height: 1;
-	margin-top: −0.47em;
+	margin-top: -0.47em;
 }
 
 .klasstheme .v-filterselect-large [class$="button"]:active:after {
@@ -3934,7 +3934,7 @@ div.v-layout.v-horizontal.v-widget {
 	text-align: center;
 	top: 50%;
 	line-height: 1;
-	margin-top: −0.47em;
+	margin-top: -0.47em;
 }
 
 .klasstheme .v-filterselect-huge [class$="button"]:active:after {
@@ -4160,7 +4160,7 @@ div.v-layout.v-horizontal.v-widget {
 	left: 0;
 	right: 0;
 	width: auto !important;
-	margin-top: −0.5em;
+	margin-top: -0.5em;
 	padding-bottom: 0.5em;
 	border-bottom: 1px solid #dfdfdf;
 }
@@ -7516,12 +7516,12 @@ div.v-layout.v-horizontal.v-widget {
 	content: "\f078";
 	font-size: 0.7em;
 	vertical-align: 0.15em;
-	margin: 0 −0.2em 0 0.5em;
+	margin: 0 -0.2em 0 0.5em;
 	opacity: 0.5;
 }
 
 .klasstheme .v-menubar .v-menubar-submenu-indicator  + .v-menubar-menuitem-caption:empty:after {
-	margin-left: −0.2em;
+	margin-left: -0.2em;
 }
 
 .klasstheme .v-menubar-popup {
@@ -8900,7 +8900,7 @@ div.v-layout.v-horizontal.v-widget {
 
 .v-ie8 .klasstheme .v-tree-node:before {
 	position: static;
-	margin-left: −1.9em;
+	margin-left: -1.9em;
 	vertical-align: top;
 	content: "\f0da";
 	font-family: ThemeIcons;
@@ -11394,7 +11394,7 @@ div.v-layout.v-horizontal.v-widget {
 	content: "\2022";
 	color: #197de1;
 	font-size: 22px;
-	margin-left: −0.07em;
+	margin-left: -0.07em;
 }
 
 .klasstheme .v-calendar .v-daterange {
@@ -11652,7 +11652,7 @@ div.v-layout.v-horizontal.v-widget {
 	margin-top: 1.4em;
 	margin-bottom: 1em;
 	
-	letter-spacing: −0.03em;
+	letter-spacing: -0.03em;
 }
 
 .klasstheme h2, .klasstheme .v-label-h2 {
@@ -11660,7 +11660,7 @@ div.v-layout.v-horizontal.v-widget {
 	
 	margin-top: 1.6em;
 	margin-bottom: 0.77em;
-	letter-spacing: −0.02em;
+	letter-spacing: -0.02em;
 }
 
 .klasstheme h3, .klasstheme .v-label-h3 {
@@ -11691,7 +11691,7 @@ div.v-layout.v-horizontal.v-widget {
 }
 
 .klasstheme .v-verticallayout > .v-slot:first-child .v-formlayout-contentcell h1, .klasstheme .v-verticallayout > .v-slot:first-child .v-formlayout-contentcell .v-label-h1, .klasstheme .v-verticallayout > .v-slot:first-child .v-formlayout-contentcell h2, .klasstheme .v-verticallayout > .v-slot:first-child .v-formlayout-contentcell .v-label-h2, .klasstheme .v-verticallayout > .v-slot:first-child .v-formlayout-contentcell h3, .klasstheme .v-verticallayout > .v-slot:first-child .v-formlayout-contentcell .v-label-h3, .klasstheme .v-verticallayout > .v-slot:first-child .v-formlayout-contentcell h4, .klasstheme .v-verticallayout > .v-slot:first-child .v-formlayout-contentcell .v-label-h4, .klasstheme .v-verticallayout > div > .v-slot:first-child .v-formlayout-contentcell h1, .klasstheme .v-verticallayout > div > .v-slot:first-child .v-formlayout-contentcell .v-label-h1, .klasstheme .v-verticallayout > div > .v-slot:first-child .v-formlayout-contentcell h2, .klasstheme .v-verticallayout > div > .v-slot:first-child .v-formlayout-contentcell .v-label-h2, .klasstheme .v-verticallayout > div > .v-slot:first-child .v-formlayout-contentcell h3, .klasstheme .v-verticallayout > div > .v-slot:first-child .v-formlayout-contentcell .v-label-h3, .klasstheme .v-verticallayout > div > .v-slot:first-child .v-formlayout-contentcell h4, .klasstheme .v-verticallayout > div > .v-slot:first-child .v-formlayout-contentcell .v-label-h4 {
-	margin-top: −0.5em;
+	margin-top: -0.5em;
 }
 
 .klasstheme h1.no-margin, .klasstheme .v-label-h1.no-margin, .klasstheme h2.no-margin, .klasstheme .v-label-h2.no-margin, .klasstheme h3.no-margin, .klasstheme .v-label-h3.no-margin, .klasstheme h4.no-margin, .klasstheme .v-label-h4.no-margin {
diff --git a/klass-shared/pom.xml b/klass-shared/pom.xml
index 5b584b10..fdeffd8c 100644
--- a/klass-shared/pom.xml
+++ b/klass-shared/pom.xml
@@ -26,8 +26,7 @@
 
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
-        <java.version>11</java.version>
-
+        <java.version>17</java.version>
         <findbugs-version>3.0.5</findbugs-version>
         <checkstyle-version>3.3.0</checkstyle-version>
         <spring.version>4.3.7.RELEASE</spring.version>
diff --git a/pom.xml b/pom.xml
index 15dc3625..32a9c735 100644
--- a/pom.xml
+++ b/pom.xml
@@ -38,7 +38,7 @@
         </snapshotRepository>
     </distributionManagement>
     <properties>
-        <java.version>11</java.version>
+        <java.version>17</java.version>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
         <findbugs-version>3.0.2</findbugs-version>
@@ -49,8 +49,10 @@
         <vaadin-spring-ext-security-version>0.0.7.RELEASE</vaadin-spring-ext-security-version>
         <objenesis-version>2.4</objenesis-version>
         <guava-version>19.0</guava-version>
+        <!-- Vaadin 7.7.17 was the last public open-source version of the Vaadin 7 framework.
+             See https://vaadin.com/support/vaadin-7-extended-maintenance  -->
         <vaadin-version>7.7.17</vaadin-version>
-        <vaadin.plugin.version>7.7.25</vaadin.plugin.version>
+        <vaadin.plugin.version>7.7.17</vaadin.plugin.version>
         <vaadin.testbench.api.version>7.7.14</vaadin.testbench.api.version>
         <vaadin.widgetset.mode>local</vaadin.widgetset.mode>
         <woodstox-version>4.4.1</woodstox-version>