1. Right click the feature class in the Contents pane and open symbology from the context menu.

2. Shift click the first and last class from the "Primary Symbology" pane to highlight all values.

3. Click the "More" button in the "Primary Symbology" pane, and choose "Format all symbols" from the context menu.

4. Switch from gallery view to properties view in the "Format Polygon Symbols" pane.

5. Adjust outline color/size.

6. hit apply and then hit the back arrow in the "format polygon symbols" pane.

This should change the outline properties but leave the fill colors intact.

Transparency can just be adjusted using the slider in the Appearance tab in the ribbon.

1. Right click the feature class in the Contents pane and open symbology from the context menu.

2. Shift click the first and last class from the "Primary Symbology" pane to highlight all values.

3. Click the "More" button in the "Primary Symbology" pane, and choose "Format all symbols" from the context menu.

4. Switch from gallery view to properties view in the "Format Polygon Symbols" pane.

5. Adjust outline color/size.

6. Click apply and then hit the back arrow in the "format polygon symbols" pane.

This should change the outline properties but leave the fill colors intact.

Transparency can just be adjusted using the slider in the Appearance tab in the ribbon.