feat: provide and consume scroll offsets

src/context/Theme.tsx

@@ -72,10 +72,14 @@ export type Theme = Overrides & {
     font?: Fonts;
     css?: string;
     monospaceFont?: MonospaceFonts;
-
     "min-opacity"?: number;
 };
 
+export type ComputedVariables = Theme & {
+    "header-height"?: string;
+    "effective-bottom-offset"?: string;
+};
+
 export interface ThemeOptions {
     base?: string;
     ligatures?: boolean;