Refactor CSS styles and introduce theme variables

- Removed redundant CSS rules and consolidated styles across dashboard, forms, imports, projects, and scenarios.
- Introduced new color variables in theme-default.css for better maintainability and consistency.
- Updated existing styles to utilize new color variables, enhancing the overall design.
- Improved responsiveness and layout of various components, including tables and cards.
- Ensured consistent styling for buttons, links, and headers across the application.

templates/base.html

@@ -4,6 +4,7 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>{% block title %}CalMiner{% endblock %}</title>
+  <link rel="stylesheet" href="/static/css/theme-default.css" />
   <link rel="stylesheet" href="/static/css/main.css" />
   <link rel="stylesheet" href="/static/css/forms.css" />
   <link rel="stylesheet" href="/static/css/imports.css" />