Can I delete that CSS comment?
No, it's not just a comment, it's a license and attribution notice for the original creator.
Even though you modified the original code, as I covered in my answer here, under Licensing conditions, the Apache License 2.0 states:
You must retain, in the Source form of any Derivative Works that You distribute, all copyright, patent, trademark, and attribution notices from the Source form of the Work...
How can I get a license for my modifications?
Simply add a license and attribution notice like the one in your question (with your name as the "creator") to any modifications or derivative works that you wish to assign a license to. You may even add different license terms and conditions as stated here:
You may add Your own copyright statement to Your modifications and may
provide additional or different license terms and conditions for use,
reproduction, or distribution of Your modifications, or for any such
Derivative Works as a whole...
Can I write "designed by me" in the footer?
As long as you maintain the license attributions and copyright notices in the code, you're free to add any generalized visible statements like this to your template, which many template and theme designers using third-party libraries & frameworks chose to do.