Refactor styles across multiple components to use new color variables

- Updated background and border colors in DashboardWidget, DialogWidget, MessageboxWidget, and TermineWidget to use new CSS variables for consistency.
- Changed button colors in various widgets and dialogs to utilize the new primary color variable.
- Adjusted text colors in VocabCourseView, VocabDictionaryView, and VocabTrainerView to align with the new color scheme.
- Replaced hardcoded colors with CSS variables in multiple components for improved maintainability and theming.

frontend/src/dialogues/chat/MultiChatDialog.vue

@@ -2439,7 +2439,7 @@ export default {
 .send-btn {
     padding: 0.3em 1.1em;
     border-radius: 3px;
-    background: #1976d2;
+    background: var(--color-primary);
     color: #fff;
     border: none;
     cursor: pointer;

frontend/src/dialogues/socialnetwork/UserProfileDialog.vue

@@ -577,7 +577,7 @@ export default {
 .image-list li {
     display: inline-block;
     padding: 2px;
-    border: 1px solid #F9A22C;
+    border: 1px solid var(--color-secondary);
     margin: 0 4px 4px 0;
 }
 
@@ -666,7 +666,7 @@ export default {
 }
 
 .activities {
-    background-color: #F9A22C;
+    background-color: var(--color-secondary);
     margin: -20px -20px 0 -20px;
     height: 26px !important;
     display: flex !important;

frontend/src/dialogues/standard/ContactDialog.vue

@@ -72,7 +72,7 @@ p {
 }
 
 a {
-    color: #007bff;
+    color: var(--color-primary);
     text-decoration: none;
 }
 

frontend/src/dialogues/standard/DataPrivacyDialog.vue

@@ -60,7 +60,7 @@ p {
 }
 
 a {
-    color: #007bff;
+    color: var(--color-primary);
     text-decoration: none;
 }
 

frontend/src/dialogues/standard/ImprintDialog.vue

@@ -60,7 +60,7 @@ p {
 }
 
 a {
-    color: #007bff;
+    color: var(--color-primary);
     text-decoration: none;
 }