fix: styles under dark mode

[DannySu09]

Hi, just found 2 places that annoyed me under dark mode, therefore I made some fixes.

Remove the default style from browsers under dark mode

I found the color-scheme: dark/light on the HTML element will apply the default style from browsers.

As the image shows below, you can see a darker gray background behind the form region, comparing the lighter gray background of the user's site.

Fix the text color of the submit button

After removing the color-scheme: dark/light style on the HTML element, the text color of the submit button will still be white under dark mode. So I added a new class dark:text-gray-100 on the button to correct the text color.