Add icon to switch to dark theme

src/App.js

@@ -11,8 +11,10 @@ import DialogContent from '@mui/material/DialogContent';
 import DialogContentText from '@mui/material/DialogContentText';
 import DialogTitle from '@mui/material/DialogTitle';
 import Divider from '@mui/material/Divider';
+import IconButton from '@mui/material/IconButton';
 import MenuItem from '@mui/material/MenuItem';
 import Select from '@mui/material/Select';
+import SettingsBrightnessIcon from '@mui/icons-material/SettingsBrightness';
 import TextField from '@mui/material/TextField';
 import Toolbar from '@mui/material/Toolbar';
 import Typography from '@mui/material/Typography';
@@ -164,7 +166,9 @@ function TopBar({toggleTheme}) {
         <Typography variant="h6" component="div" sx={{ flexGrow: 1 }}>
           ENcarteS
         </Typography>
-        <Button color="inherit" onClick={toggleTheme}>Changer de thème</Button>
+        <IconButton onClick={toggleTheme} size="large">
+          <SettingsBrightnessIcon fontSize="inherit"/>
+        </IconButton>
         <Button color="inherit" onClick={handleOpenAboutModal}>À propos</Button>
       </Toolbar>
     </AppBar>