x11-themes/materia-gtk-theme: Update to 20200320

From ChangeLog: https://github.com/nana-4/materia-theme/releases/tag/v20200320

• Make dark variant even darker
• Don't change button text color on focus, hover, active
• gtk: Rewrite ripple implementation
• gtk: Change checked raised button style
• gtk: Lighten up switch slider color on dark variant
• gtk: Use primary color for spinner
• gtk: Use primary color for caret-color (experimentally)
• gtk: Add various improvements of Budgie styling
• gnome-shell: Add support for version 3.36
• gnome-shell: Drop support for versions 3.18 - 3.24
• gnome-shell: Drop support for legacy Ubuntu-specific sessions
• gnome-shell: Land massive Sass rewrite
• Other various improvements, fixes, refactors, and cleanups

PR: 244944
Submitted by: tagattie@yandex.com (maintainer)