Using a Dash Bootstrap theme changes the layout of my app

I created an app using dash core components and I want to style them using Bootstrap themes. But when I apply a theme using app = dash.Dash(__name__, external_stylesheets=[dbc.themes.BOOTSTRAP]) my app layout is messed up. My original dashboard looked like this:

In this I used:

 external_stylesheets = ['']
app = dash.Dash(__name__, external_stylesheets=external_stylesheets)

But after applying the bootstrap theme my app looks like:

What can I do to avoid these changes? I don’t want to rewrite the whole app using bootstrap components!