Allow the Login screen to use the dark theme (#31293)

* Allow the Login screen to use the dark theme

There is a whole bunch of code dedicated to attempting to force a "light" theme
onto the login view, even if the deployment is configured to prioritise the
dark theme.

Apparently this was done because, in the old days, the
login view looked rubbish with the dark theme, but now it looks fine.

Fixes https://github.com/element-hq/element-web/issues/31292

* Fix up background colour of auth screen in dark theme

In dark theme, the background of the login/register/welcome box needs to be
translucent black rather than translucent white.

To make this work, I've moved the style from inline to CSS, and used a PostCSS
variable to define the colour.

* Fix up Welcome page colours in dark theme

Make the text and image white, not black, in short.

* Fix incorrect merge

that will teach me to use the Github UI to address merge conflicts.

test/unit-tests/components/structures/__snapshots__/MatrixChat-test.tsx.snap

@@ -123,7 +123,7 @@ exports[`<MatrixChat /> Multi-tab lockout waits for other tab to stop during sta
       <main
         aria-live="polite"
         class="mx_AuthPage_modalContent"
-        style="display: flex; z-index: 1; border-radius: inherit; background: rgba(255, 255, 255, 0.59);"
+        style="display: flex; z-index: 1; border-radius: inherit;"
         tabindex="-1"
       >
         <div
@@ -243,7 +243,7 @@ exports[`<MatrixChat /> with a soft-logged-out session should show the soft-logo
       <main
         aria-live="polite"
         class="mx_AuthPage_modalContent"
-        style="display: flex; z-index: 1; border-radius: inherit; background: rgba(255, 255, 255, 0.59);"
+        style="display: flex; z-index: 1; border-radius: inherit;"
         tabindex="-1"
       >
         <div