Make dark theme hrs consistent with light theme

[lyubomir-popov]

There's a difference in opacity between hr and muted hr colours between the themes. Can we please use the following values
$colors--dark-theme--border-default: rgba($colors--dark-theme--text-default, 0.3); $colors--dark-theme--border-low-contrast: rgba($colors--dark-theme--text-default, 0.1) !default;

[bartaz]

@lyubomir-popov Isn't this what you already fixed in this PR? https://github.com/canonical/vanilla-framework/pull/5056/files