Improve contrast on blame timestamp, fix double border (#29482)

Improve contrast on blame timestamp, fix double border (#29482)

Before, double border on top, bad contrast on dark:
<img width="155" alt="Screenshot 2024-02-29 at 02 06 17"
src="https://github.com/go-gitea/gitea/assets/115237/fc0f1e08-a5ce-47ed-9eb6-135eed5a1abb">
<img width="126" alt="Screenshot 2024-02-29 at 02 07 28"
src="https://github.com/go-gitea/gitea/assets/115237/38ae8483-8d9b-484c-8909-d4466131ea16">

After, no double border on top, good contrast:
<img width="154" alt="Screenshot 2024-02-29 at 02 20 20"
src="https://github.com/go-gitea/gitea/assets/115237/ad91282b-e9f5-4f41-8f5e-6ba28db3beac">
<img width="147" alt="Screenshot 2024-02-29 at 02 20 38"
src="https://github.com/go-gitea/gitea/assets/115237/7ee2ec92-e72a-4981-aec3-98fc8e579bae">