How can I change all my layout to my desired theme?
go into layout mode on the layout you want to change.
In the top menu choose Layouts...Change Theme
As far as I'm aware, the only way to do this is if you change the actual theme itself. If you use Layout, Change theme, it acts on the current layout as you've seen.
When starting, you could create a custom theme based on one of the existing themes. This would mean it's more personal to your preferences.
One of the dangers when changing to a complete different them is that the actual layouts may appear different which may in turn change the way the layout looks overall. For example, some items may 'disappear' of the page.
Yes, you have to change the theme for each layout and you are correct each theme is different so it can change the position of items, which is one reason you have to change the theme on each layout. Another reason is because it is normal to have several different themes in one database, for example a report theme may use the enlightened print theme.
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