Alternatively, for a more robust solution, you could use templates.

in your application/blocks folder, create a folder labelled image_slider just like the image slider block folder. Inside that folder create another one called templates and inside templates create another folder that you can call anything you want following those rules: all small caps and use underscore _ instead of spaces to something like my_template or custom_size

Then go to the original image slider found in concrete/blocks/image_slider and copy both files view.php and view.css

Paste a copy of each in the folder you created in the first step. So now you have application/blocks/image_slider/templates/your_template_folder/view.php and application/blocks/image_slider/templates/your_template_folder/view.css

From here you can modify the css in your template.

Then go to your page and put it in edit mode, click on your slider and select "design & custom template". From the toolbar that appears, click on the little cog icon and from the popup that appears, under "custom template" you select your template, then click on the blue save button and finally publish your page.