How to structure a presentation in Impress libreoffice

Press F11, you will see a style dialog. Right click on "Default", and change the font.

I can't find the setting to make rounded corners.

Using Customize Toolbar, you can add the Rectangles toolbar.

Rounded Rectangle

Draws a rounded rectangle with a fill where you drag in the current document. Click where you want to place a corner of the rounded rectangle, and drag to the size you want. To draw a rounded square, hold down Shift while you drag.

Rounded Square

Draws a rounded square with a fill where you drag in the current document. Click where you want to place a corner of the rounded square, and drag to the size you want. To draw a rounded rectangle, hold down Shift while you drag.

Source Rectangles

When you insert a rectangle or a callout box using the drawing functions and activate the Points icon on the Drawing toolbar, you see a small frame at the upper left corner of the object.

The frame indicates the amount by which the corners are rounded. When the frame is positioned at the top left corner, no rounding occurs.

When the frame is positioned on the handle centered at the top of the object, the corners are rounded as much as possible.

You adjust the degree of rounding by moving the frame between these two positions.

If you place the cursor on the box it changes to a hand symbol . You can now drag the box to change the amount of rounding. An outline shows a preview of the result.

Source Creating Round Corners