change the rotation angle by any degree
This is a feature request.
gThum allows rotation by fixed amounts (90°, etc.) but not by a "free" amount, which is essential if you want to set some images straight.
Darktable has a crop module where you can also set the angle (in percentage) you want the image to rotate. If you right-click on the image and drag a vertical or horizontal line, the image will be rotated to match that angle.
Darktable user manual explains it better:
angle
This tool corrects the rotation angle helping you level an image. You can either set a numerical value or use your mouse directly on the image. To use your mouse, right-click, hold it down and draw a line along a suited horizontal or vertical feature; as soon as you release the mouse button the image is rotated so the line you drew matches the horizontal/vertical axis.