On this page of the "Colors and line styles" dialog (which can be opened using the corresponding button in the toolbar at the top of the main window), you can set the colors of all kinds of graphical elements in the diffraction pattern, like background color, border line, grid color etc., as well as the font style and size for the texts and numbers in the pattern graphics.
For most graphical elements, the current color is displayed in the corresponding box. On the right-hand side of each color box you will find a button "Modify...". By pressing on the button you can open a color selection box where you can select/define a new color for the corresponding graphical element.
In many cases, it is not only possible to define the color but also the line style and width of the corresponding graphical element. The current line style is displayed to the right of the "Modify..." button; if you press on the small arrow on its right-hand side, you will get a list of available line styles from which you can also select the new line style.
To the right of the line style box, you can enter or adjust the corresponding line width (given in points). Either use the "up" and "down" array buttons to modify the line width, or enter a new value directly.
At the bottom of the "Pattern graphics" page, you can define the font style and size for all texts and numbers in the diffraction pattern, simply by clicking the on the arrows on the right-hand side of the corresponding selection boxes.