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/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;
 }