Add transparancy to modal background for accessibility (#18081)

Fixes #18080 

This keeps the `ui-base-lighter-color` but adds enough transparency so that text is more easily readable.
Tested in Firefox and Chrome.

app/javascript/styles/mastodon/modal.scss

 .modal-layout {
-  background: $ui-base-color url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 234.80078 31.757813" width="234.80078" height="31.757812"><path d="M19.599609 0c-1.05 0-2.10039.375-2.90039 1.125L0 16.925781v14.832031h234.80078V17.025391l-16.5-15.900391c-1.6-1.5-4.20078-1.5-5.80078 0l-13.80078 13.099609c-1.6 1.5-4.19883 1.5-5.79883 0L179.09961 1.125c-1.6-1.5-4.19883-1.5-5.79883 0L159.5 14.224609c-1.6 1.5-4.20078 1.5-5.80078 0L139.90039 1.125c-1.6-1.5-4.20078-1.5-5.80078 0l-13.79883 13.099609c-1.6 1.5-4.20078 1.5-5.80078 0L100.69922 1.125c-1.600001-1.5-4.198829-1.5-5.798829 0l-13.59961 13.099609c-1.6 1.5-4.200781 1.5-5.800781 0L61.699219 1.125c-1.6-1.5-4.198828-1.5-5.798828 0L42.099609 14.224609c-1.6 1.5-4.198828 1.5-5.798828 0L22.5 1.125C21.7.375 20.649609 0 19.599609 0z" fill="#{hex-color($ui-base-lighter-color)}"/></svg>') repeat-x bottom fixed;
+  background: $ui-base-color url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 234.80078 31.757813" width="234.80078" height="31.757812"><path d="M19.599609 0c-1.05 0-2.10039.375-2.90039 1.125L0 16.925781v14.832031h234.80078V17.025391l-16.5-15.900391c-1.6-1.5-4.20078-1.5-5.80078 0l-13.80078 13.099609c-1.6 1.5-4.19883 1.5-5.79883 0L179.09961 1.125c-1.6-1.5-4.19883-1.5-5.79883 0L159.5 14.224609c-1.6 1.5-4.20078 1.5-5.80078 0L139.90039 1.125c-1.6-1.5-4.20078-1.5-5.80078 0l-13.79883 13.099609c-1.6 1.5-4.20078 1.5-5.80078 0L100.69922 1.125c-1.600001-1.5-4.198829-1.5-5.798829 0l-13.59961 13.099609c-1.6 1.5-4.200781 1.5-5.800781 0L61.699219 1.125c-1.6-1.5-4.198828-1.5-5.798828 0L42.099609 14.224609c-1.6 1.5-4.198828 1.5-5.798828 0L22.5 1.125C21.7.375 20.649609 0 19.599609 0z" fill="#{hex-color($ui-base-lighter-color)}33"/></svg>') repeat-x bottom fixed;
   display: flex;
   flex-direction: column;
   height: 100vh;