docs(theme): add first draft doc for theme

Author: Junjiequan

docs/theme/color-theme.md

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+---
+title: Theme System Overview & New Variables
+audience: Technical
+created_by: Junjie Quan
+created_on: 2025/12/05
+---
+
+# Theme System Overview
+
+The SciCat front-end uses a **CSS variable–based theming system**.  
+All configurable visual properties—colors, and component-specific styles—are exposed as `--theme-*` variables.  
+
+This allows deployments to adjust appearance without touching source code.
+The application reads these variables directly in SCSS using standard var() usage:
+
+```scss
+color: var(--theme-header-1-default);
+border: var(--theme-table-border);
+box-shadow: var(--theme-table-shadow);
+```
+
+# Variable Details
+
+`--theme-primary-default`
+
+`--theme-primary-default-contrast`
+
+… Above variables control general old theme colors (modify as needed)
+
+
+
+# Newly Added Variables
+
+### Date: 2025-12-05
+
+```scss
+--theme-side-panel-border
+--theme-side-panel-shadow
+--theme-table-border
+--theme-table-shadow
+```
+⚠️ _NOTE: The 4 variables are currently implemented only on the Datasets and Proposals pages._
+
+`--theme-side-panel-border`:
+Controls the border around the collapsible side panel.
+- Default: none
+- Component-level fallback:
+  -  1px solid rgba(153, 213, 233, 0.7) (provided through var(--theme-table-border, fallback) in SCSS)
+- Set to none to remove the border completely.
+
+`--theme-side-panel-shadow`:
+Controls the shadow applied to the side panel.
+- Default: Material-like elevation shadow
+  - 0px 2px 1px -1px rgba(0, 0, 0, 0.20), 0px 1px 1px 0px rgba(0, 0, 0, 0.14), 0px 1px 3px 0px rgba(0, 0, 0, 0.12)`
+- Set to none to disable shadows.
+
+`--theme-table-border`:
+Controls the border around the table container.
+- Default: none
+- Component-level fallback:
+  -  1px solid rgba(153, 213, 233, 0.7) (provided through var(--theme-table-border, fallback) in SCSS)
+- Set to none to remove the border completely.
+
+`--theme-table-shadow`:
+Controls the shadow applied to the table container.
+- Default: Material-like elevation shadow
+  - 0px 2px 1px -1px rgba(0, 0, 0, 0.20), 0px 1px 1px 0px rgba(0, 0, 0, 0.14), 0px 1px 3px 0px rgba(0, 0, 0, 0.12)`
+- Set to none to disable shadows.