diff --git a/docs/src/components/footer/EmailSubscribe.tsx b/docs/src/components/footer/EmailSubscribe.tsx
index 1f7d106296e8c2..44503af17abacf 100644
--- a/docs/src/components/footer/EmailSubscribe.tsx
+++ b/docs/src/components/footer/EmailSubscribe.tsx
@@ -137,12 +137,12 @@ export default function EmailSubscribe({ sx }: { sx?: SxProps<Theme> }) {
             },
 
             [`&.${inputBaseClasses.focused}`]: {
-              borderColor: (theme) =>
+              borderColor: (theme: Theme) =>
                 theme.palette.mode === 'dark'
                   ? theme.palette.primaryDark[300]
                   : theme.palette.primary[500],
               outline: '3px solid',
-              outlineColor: (theme) =>
+              outlineColor: (theme: Theme) =>
                 theme.palette.mode === 'dark'
                   ? theme.palette.primaryDark[500]
                   : theme.palette.primary[200],
diff --git a/packages/mui-system/src/styleFunctionSx/styleFunctionSx.d.ts b/packages/mui-system/src/styleFunctionSx/styleFunctionSx.d.ts
index 5ab2389193da52..b59c327d3668c9 100644
--- a/packages/mui-system/src/styleFunctionSx/styleFunctionSx.d.ts
+++ b/packages/mui-system/src/styleFunctionSx/styleFunctionSx.d.ts
@@ -26,7 +26,12 @@ export interface CSSSelectorObject<Theme extends object = {}> {
 
 type CssVariableType = string | number;
 
-type AllowedKeys = `--${string}` | `&${string}`;
+type AllowedKeys =
+  | `--${string}`
+  | `&${string}`
+  | `${CSS.AtRules}${string}`
+  | `::${string}`
+  | `${number}%`;
 
 /**
  * Map all nested selectors and CSS variables.