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.

static/css/theme-default.css

@@ -0,0 +1,72 @@
+:root {
+  /* Neutral surfaces */
+  --color-bg-base: #0b0f14;
+  --color-bg-elevated: #0f141b;
+  --color-surface-default: #151b23;
+  --color-surface-overlay: rgba(21, 27, 35, 0.7);
+
+  --color-border-subtle: rgba(255, 255, 255, 0.08);
+  --color-border-card: rgba(255, 255, 255, 0.08);
+  --color-border-strong: rgba(255, 255, 255, 0.12);
+  --color-highlight: rgba(241, 178, 26, 0.08);
+
+  /* Text */
+  --color-text-primary: #e6edf3;
+  --color-text-muted: #a9b4c0;
+  --color-text-subtle: rgba(169, 180, 192, 0.6);
+  --color-text-invert: #ffffff;
+  --color-text-dark: #0f172a;
+  --color-text-strong: #111827;
+
+  /* Brand & accent */
+  --color-brand-base: #f1b21a;
+  --color-brand-bright: #f6c648;
+  --color-brand-soft: #f9d475;
+  --color-accent: #2ba58f;
+
+  /* Semantic states */
+  --color-success: #0c864d;
+  --color-info: #0b3d88;
+  --color-warning: #f59e0b;
+  --color-danger: #7a1721;
+
+  /* Shadows & depth */
+  --shadow: 0 10px 30px rgba(0, 0, 0, 0.35);
+  --color-panel-shadow: rgba(0, 0, 0, 0.25);
+  --color-panel-shadow-deep: rgba(0, 0, 0, 0.35);
+
+  /* Buttons */
+  --btn-primary-bg: var(--color-brand-bright);
+  --btn-primary-color: var(--color-text-dark);
+  --btn-primary-hover: var(--color-brand-soft);
+
+  --btn-secondary-bg: rgba(21, 27, 35, 0.85);
+  --btn-secondary-hover: rgba(21, 27, 35, 0.95);
+  --btn-secondary-border: var(--color-border-strong);
+  --btn-secondary-color: var(--color-text-primary);
+
+  --btn-danger-bg: var(--color-danger);
+  --btn-danger-color: var(--color-text-invert);
+  --btn-danger-hover: #a21d2b;
+
+  --btn-link-color: var(--color-brand-bright);
+  --btn-link-hover: var(--color-brand-soft);
+  --btn-ghost-color: var(--color-text-muted);
+
+  /* Legacy aliases */
+  --bg: var(--color-bg-base);
+  --bg-2: var(--color-bg-elevated);
+  --card: var(--color-surface-default);
+  --text: var(--color-text-primary);
+  --muted: var(--color-text-muted);
+  --brand: var(--color-brand-base);
+  --brand-2: var(--color-brand-bright);
+  --brand-3: var(--color-brand-soft);
+  --accent: var(--color-accent);
+  --success: var(--color-success);
+  --danger: var(--color-danger);
+  --info: var(--color-info);
+  --color-border: var(--color-border-subtle);
+  --card-border: var(--color-border-card);
+  --color-surface-alt: var(--color-surface-overlay);
+}