Set base for settings, add volatile settings for theme and language

Goal is to have a common and reusable base for the settings. Having nice styles and saving preferences is out of scope for now.

Note the conversation page is now the "main page" instead of the account settings.

Change-Id: I328686047d924642ae6781096c9f57308e8fea22
diff --git a/client/src/themes/ThemeDemonstrator.tsx b/client/src/themes/ThemeDemonstrator.tsx
index daddbee..9fb7599 100644
--- a/client/src/themes/ThemeDemonstrator.tsx
+++ b/client/src/themes/ThemeDemonstrator.tsx
@@ -15,7 +15,7 @@
  * License along with this program.  If not, see
  * <https://www.gnu.org/licenses/>.
  */
-import { Button, Stack, Switch, ThemeProvider, Typography } from '@mui/material';
+import { Button, Stack, Switch, Typography } from '@mui/material';
 
 import {
   BackButton,
@@ -28,11 +28,11 @@
   UploadPictureButton,
 } from '../components/Button';
 import { NickNameInput, PasswordInput, RegularInput, UsernameInput } from '../components/Input';
-import defaultTheme from './Default';
+import CustomThemeProvider from '../contexts/CustomThemeProvider';
 
 export const ThemeDemonstrator = () => {
   return (
-    <ThemeProvider theme={defaultTheme}>
+    <CustomThemeProvider>
       <Stack spacing="5px">
         <Stack>
           <Typography variant="h1">Exemple de titre H1</Typography>
@@ -78,6 +78,6 @@
           <RegularInput />
         </Stack>
       </Stack>
-    </ThemeProvider>
+    </CustomThemeProvider>
   );
 };