[core] DashboardLayout theme overrides issues

[bharatkashyap]

1. Many styles from List and ListItem not being applied to Toolpad Core layout

Styling seems to be working differently from the current Material UI components.In the custom theme, there are many styles for List and ListItem , but these aren’t being applied to the layout.

Toolpad Core Theme app	Material UI Dashboard Template App

2. Custom theme breaking for focus and active states

The custom theme seems to be applied in most of the UI, but in some states such as focus and active are acting weird:

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[apedroferreira]

Some of these styles are set so that the components look nicer by default, I expected them to be easily overridable in the theme.
If they're not that straightforward to override we can try maybe not setting some of them, not sure - maybe the app will still look ok enough with the default styles, but would have to check.
Otherwise I guess the template theme might be missing some styles that are needed to be used on top of Toolpad Core.