To change the map border color, style or line weight you can use the Stroke panel.
Specifying Map Borders color, style, and weight in the Stroke panel
1. Open a FLA-file, unlock the layer titled "borders" and click on it to select all borders.
To select a stroke color, style, and weight with the Stroke panel:
2. Choose Window > Panels > Stroke.
3. To select a color, click the triangle next to the Stroke color box and do one of the following:
3.1. Choose a color swatch from the palette.
3.2. Type a color's hexadecimal value in the text box.
4. To select a stroke style, click the triangle next to the Style pop-up menu and choose an option from the menu. To create a custom style, choose Custom from the pop-up menu in the upper right corner of the Stroke panel, then choose options in the Line Style dialog box and click OK.
5. To select a stroke weight, click the triangle next to the Weight pop-up menu and set the slider at the desired weight.
6. Lock the "borders" layer.