diff --git a/components/templates/styleguide/_styled_tag4.ts b/components/templates/styleguide/_styled_tag4.ts
index b6a03811..063ae8ae 100644
--- a/components/templates/styleguide/_styled_tag4.ts
+++ b/components/templates/styleguide/_styled_tag4.ts
@@ -1,3 +1,3 @@
 import styled from 'styled-components'
 
-export const StyledTag4 = styled.div`border: 4px solid black;`
+export const StyledTag4 = styled.div`border: 4px solid ${({ theme }) => theme.colors.primary};`
diff --git a/components/templates/styleguide/index.tsx b/components/templates/styleguide/index.tsx
index 69ca4ed6..64c06786 100644
--- a/components/templates/styleguide/index.tsx
+++ b/components/templates/styleguide/index.tsx
@@ -2,6 +2,7 @@ import { Index2 } from './index2'
 import { FooBar, FooBarTag4 } from './_foo_bar'
 import styled, { css, keyframes, Keyframes } from 'styled-components'
 import { StyledTag4 } from './_styled_tag4'
+import { useSetAppState, useAppState } from '@/hooks/app_state'
 
 const hover = css`
   &:hover {
@@ -17,6 +18,8 @@ const prop = css`
 `
 
 const Styleguide = () => {
+  const setAppState = useSetAppState()
+  console.log(useAppState())
   return (
     <>
       <Index2 />
@@ -31,6 +34,12 @@ const Styleguide = () => {
       <StyledTag3 animation={ bounce }>Let&apos;s bounce.</StyledTag3>
       <StyledTag3 animation={ shake }>Let&apos;s shake.</StyledTag3>
       <StyledTag4>This is Styled.Tag4</StyledTag4>
+      <button onClick={ () => setAppState({ themeName: 'dark' }) }>
+        dark mode
+      </button>
+      <button onClick={ () => setAppState({ themeName: 'light' }) }>
+        light mode
+      </button>
     </>
   )
 }
@@ -90,4 +99,5 @@ const StyledTag3 = styled.div<{ animation: Keyframes }>`
     background-color: linen;
   }
   animation: ${({ animation }) => animation} 0.2s infinite ease-in-out alternate;
+  content: "${({ theme }) => theme.colors.primary}";
 `
diff --git a/contexts/app_state.tsx b/contexts/app_state.tsx
new file mode 100644
index 00000000..ca3b4d88
--- /dev/null
+++ b/contexts/app_state.tsx
@@ -0,0 +1,41 @@
+import { createContext, Dispatch, SetStateAction, useState } from 'react'
+import { ThemeProvider } from 'styled-components'
+import { lightTheme, darkTheme } from '@/styles/app_theme'
+
+interface AppState {
+  // TODO: Add null
+  themeName: 'light' | 'dark'
+}
+
+const initialAppState: AppState = {
+  themeName: 'light'
+}
+
+export const AppStateContext = createContext<AppState>(initialAppState)
+export const SetAppStateContext = createContext<
+Dispatch<SetStateAction<AppState>>
+>(() => {})
+
+export const AppStateProvider = (props: {
+  initialAppState?: AppState
+  children: React.ReactNode
+}) => {
+  const [appState, setAppState] = useState<AppState>(
+    props.initialAppState ?? initialAppState
+  )
+  return (
+    <AppStateContext.Provider value={ appState }>
+      <SetAppStateContext.Provider value={ setAppState }>
+        <AppStateContext.Consumer>
+          { value => (
+            <ThemeProvider theme={ value.themeName === 'light'
+              ? lightTheme : darkTheme
+            }>
+              { props.children }
+            </ThemeProvider>
+          ) }
+        </AppStateContext.Consumer>
+      </SetAppStateContext.Provider>
+    </AppStateContext.Provider>
+  )
+}
diff --git a/hooks/app_state.tsx b/hooks/app_state.tsx
new file mode 100644
index 00000000..8e5251d7
--- /dev/null
+++ b/hooks/app_state.tsx
@@ -0,0 +1,10 @@
+import { useContext } from 'react'
+import { AppStateContext, SetAppStateContext } from '@/contexts/app_state'
+
+export const useAppState = () => {
+  return useContext(AppStateContext)
+}
+
+export const useSetAppState = () => {
+  return useContext(SetAppStateContext)
+}
diff --git a/pages/_app.tsx b/pages/_app.tsx
index 6b34fb46..6b0e8ea9 100644
--- a/pages/_app.tsx
+++ b/pages/_app.tsx
@@ -1,26 +1,23 @@
 import { AppProps } from 'next/app'
 import Router from 'next/router'
-import { ThemeProvider, DefaultTheme } from 'styled-components'
 import * as gtag from '@/utils/gtag'
 import Styles from '@/styles'
+import { AppStateProvider } from '@/contexts/app_state'
+import { useAppState } from '@/hooks/app_state'
 import '@/styles/normalize.scss'
 import '@/styles/highlightjs.scss'
 import '@/styles/fonts.scss'
 
 Router.events.on('routeChangeComplete', url => gtag.pageview(url))
 
-const theme: DefaultTheme = {
-  colors: {
-    primary: 'skyblue'
-  }
-}
-
 export default function App ({ Component, pageProps }: AppProps) {
+  const appState = useAppState()
+
   return (
-    <ThemeProvider theme={ theme }>
+    <AppStateProvider initialAppState={ appState }>
       <Styles.CustomProperties />
       <Styles.Elements />
       <Component { ...pageProps } />
-    </ThemeProvider>
+    </AppStateProvider>
   )
 }
diff --git a/styles/app_theme.ts b/styles/app_theme.ts
new file mode 100644
index 00000000..868526b0
--- /dev/null
+++ b/styles/app_theme.ts
@@ -0,0 +1,46 @@
+export interface AppTheme {
+  colors: {
+    primary: string
+  }
+  sizes: {
+    font: {
+      XS: number
+      SM: number
+      MD: number
+      LG: number
+      XL: number
+    }
+    width: {
+      CONTENT: number
+    }
+  }
+}
+
+export const lightTheme: AppTheme = {
+  colors: {
+    primary: 'skyblue'
+  },
+  sizes: {
+    font: {
+      XS: 12,
+      SM: 14,
+      MD: 16,
+      LG: 18,
+      XL: 20
+    },
+    width: {
+      CONTENT: 1000
+    }
+  }
+} as const
+
+export const darkTheme: AppTheme = {
+  ...lightTheme,
+  colors: {
+    primary: 'pink'
+  }
+} as const
+
+declare module 'styled-components' {
+  interface DefaultTheme extends AppTheme {}
+}
diff --git a/types/styled.d.ts b/types/styled.d.ts
index 772f4342..30b75d6d 100644
--- a/types/styled.d.ts
+++ b/types/styled.d.ts
@@ -1,13 +1,2 @@
-import 'styled-components'
-import { fiboMap } from '@/styles/abstractions/funcs'
-
-declare module 'styled-components' {
-  export interface DefaultTheme {
-    colors: {
-      primary: string
-    }
-  }
-}
-
 export type TFiboSizeName = 'xl6'|'xl5'|'xl4'|'xl3'|'xl2'|'xl'|'lg'|'md'|'sm'|'xs'|'xs2'|'xs3'|'xs4'|'xs5'|'xs6'
 export type TFiboUnit = 'px'|'rem'|'alpha'