Legacy CSS property rgba is used in error pages.

[ryohidaka]

Goals

1. Replace rgba with rgb where transparent color values are used.
2. Because of the large number of locations used, only error pages will be covered.

Non-Goals

No response

Background

Since rgba is a legacy notation, I suggest replacing it with rgb.

• https://developer.mozilla.org/en-US/docs/Web/CSS/color_value/rgb

Although they are currently compatible, it would be safer in the future if they were replaced.
( Especially those included in the core of the package, such as error pages )

Proposal

The current notation is rgb(R G B / A), separated by spaces.
However, to align with existing implementations, I suggest that rgb(R, G, B, A) is a better implementation.