Add popover colors to :root for inline style access

CSS variables in @theme are only for Tailwind utilities. For inline styles
with var(), variables must also be defined in :root.

Added to :root:
- --color-popover: hsl(0 0% 18%)
- --color-popover-foreground: hsl(0 0% 83%)

Now var(--color-popover) in inline styles will work correctly, ensuring
tooltips have proper background and text colors.

Author: kylecarbs

src/styles/globals.css

@@ -203,6 +203,8 @@
   --color-interrupted: hsl(38 92% 50%);
   --color-code-bg: hsl(0 6.43% 8.04%);
   --color-border: hsl(240 2% 25%);
+  --color-popover: hsl(0 0% 18%);
+  --color-popover-foreground: hsl(0 0% 83%);
   --color-text: hsl(0 0% 83%);
   --color-text-secondary: hsl(0 0% 42%);
   --color-user-border: hsl(0 0% 38%);