feat: various UI improvements (#36)

* fix: content offset

When the appbar was changed to a `div` the content became offset, this fixes that.

* style: make display of cheques table more readable

* style: restyle sidebar

* fix: content overflow

* chore: split theme into separate file

* feat: show ethereum transaction link for cashout

* feat: make cashout link to etherscan transaction

Co-authored-by: Vojtech Simetka <vojtech@simetka.cz>

src/layout/Dashboard.tsx

@@ -13,7 +13,6 @@ const useStyles = makeStyles((theme: Theme) =>
     toolbar: theme.mixins.toolbar,
     content: {
       marginLeft: '240px',
-      marginTop: '64px',
       flexGrow: 1,
       backgroundColor: theme.palette.background.default,
       padding: theme.spacing(3),