Note text box is dark theme even on light theme

[zeebok]

Expected behavior: On light theme, the text area for the note contents should match other text boxes.

Current behavior: When on light theme, the text area of a new note is dark themed

[ryonakano]

This might be help:

https://github.com/ryonakano/konbucase/blob/f5384d38a216f6f65c59235709fa31683e74c87a/src/Widgets/ComboEntry.vala#L149-L157

https://github.com/ryonakano/konbucase/blob/f5384d38a216f6f65c59235709fa31683e74c87a/src/MainWindow.vala#L107-L110