I am writing a small guide on how to remove the black bars on the side of the screen when you go full screen, due to Onigiri's resolution being lower than your native resolution.
It's very simple, here goes.

For Intel:

1. Right click on your desktop and choose graphics properties.

2. Choose Basic, and hit OK.

3. Go to resolution and pick a resolution lower than your current one.

4. Go to scaling and choose full screen, hit apply.

5. Go back to resolution and pick the one you like having your pc at, hit apply and then ok.

6. Load Onigiri and enjoy the full screen without bars.

For ATI:

1. Go to Catalyst Control Center. AMD Vision, choose the lowest possible resolution.

2. Go to my My Built-In Displays, Properties, and choose Full Screen on Scaling Options, hit apply.

3. Return to your native resolution, hit apply, and then close the control center.

4. Load Onigiri and enjoy the full screen without bars.

For NVidia:

1. Right-click on desktop and choose Nvidia Control Panel.

2. Go to the Display tab and choose Adjust desktop size and positioning.

3. Choose under scaling, Full Screen and perform scaling on: choose display. Apply, and exit the control panel.

4. Load Onigiri and enjoy the full screen without bars.

Edit: I hope this helped, and let me know if you have any questions.