scss: migrate @import -> @use, lighten/darken -> color.adjust (dart-sass)

Silences the dart-sass deprecation warnings that flooded every build:
- All @import of local partials (colors, mixins, and the CSS-emitting
  partials) -> @use '...' as * (partials are pure defs, so 'as *' keeps the
  global names and the ~25 call sites unchanged). Added explicit @use './colors'
  to bootstrap-overrides.scss and cookies-eu.scss, which used -palette*
  transitively (broke under @use's scoping).
- lighten(c, x%) -> color.adjust(c, $lightness: x%) and darken -> negative,
  with @use 'sass:color' in the 6 affected files.
Compiles clean (byte-for-byte CSS behavior preserved).

imports/ui/components/AccountPageFooter/AccountPageFooter.scss

@@ -1,4 +1,4 @@
-@import '../../stylesheets/colors';
+@use '../../stylesheets/colors' as *;
 
 .AccountPageFooter {
   margin: 25px 0 0;

imports/ui/components/Content/Content.scss

@@ -1,4 +1,4 @@
-@import '../../stylesheets/mixins';
+@use '../../stylesheets/mixins' as *;
 
 .Content {
   max-width: 700px;

imports/ui/components/DistanceSlider/DistanceSlider.scss

@@ -1,4 +1,4 @@
-@import '../../stylesheets/mixins';
+@use '../../stylesheets/mixins' as *;
 
 .dist-slider {
     width: 400px;

imports/ui/components/Feedback/Feedback.scss

@@ -1,5 +1,5 @@
 /* From: https://github.com/spektom/bootstrap-feedback-form */
-@import '../../stylesheets/mixins';
+@use '../../stylesheets/mixins' as *;
 
 #feedback {
   position: fixed;

imports/ui/components/Footer/Footer.scss

@@ -1,5 +1,5 @@
-@import '../../stylesheets/mixins';
-@import '../../stylesheets/colors';
+@use '../../stylesheets/mixins' as *;
+@use '../../stylesheets/colors' as *;
 
 html {
   position: relative;

imports/ui/components/InputHint/InputHint.scss

@@ -1,4 +1,4 @@
-@import '../../stylesheets/colors';
+@use '../../stylesheets/colors' as *;
 
 .InputHint {
   display: block;

imports/ui/components/Navigation/Navigation.scss

@@ -1,4 +1,4 @@
-@import '../../stylesheets/mixins';
+@use '../../stylesheets/mixins' as *;
 
 .navbar {
   border-radius: 0;

imports/ui/components/OAuthLoginButton/OAuthLoginButton.scss

@@ -1,4 +1,5 @@
-@import '../../stylesheets/colors';
+@use 'sass:color';
+@use '../../stylesheets/colors' as *;
 
 .OAuthLoginButton {
 /*  display: block;
@@ -19,21 +20,21 @@
     background: $facebook;
     color: #fff;
 
-    &:hover { background: darken($facebook, 5%); }
+    &:hover { background: color.adjust($facebook, $lightness: -5%); }
   }
 
   &.OAuthLoginButton-google {
     background: $google;
     color: #fff;
 
-    &:hover { background: darken($google, 5%); }
+    &:hover { background: color.adjust($google, $lightness: -5%); }
   }
 
   &.OAuthLoginButton-github {
     background: $github;
     color: #fff;
 
-    &:hover { background: darken($github, 5%); }
+    &:hover { background: color.adjust($github, $lightness: -5%); }
   }
 
   &:active {

imports/ui/components/OAuthLoginButtons/OAuthLoginButtons.scss

@@ -1,4 +1,5 @@
-@import '../../stylesheets/colors';
+@use 'sass:color';
+@use '../../stylesheets/colors' as *;
 
 .OAuthLoginButtons {
   margin-bottom: 25px;
@@ -11,7 +12,7 @@
     .EmailMessage {
       display: inline-block;
       background: #fff; /* default */
-      background: lighten($todos-palette5, 70%);
+      background: color.adjust($todos-palette5, $lightness: 70%);
       padding: 0 10px;
       position: absolute;
       bottom: -27px;

imports/ui/components/PageHeader/PageHeader.scss

@@ -1,5 +1,5 @@
-@import '../../stylesheets/mixins';
-@import '../../stylesheets/colors';
+@use '../../stylesheets/mixins' as *;
+@use '../../stylesheets/colors' as *;
 
 .PageHeader {
   border-bottom: 1px solid $gray-lighter;

imports/ui/components/SelectionMap/SelectionMap.scss

@@ -1,4 +1,4 @@
-@import '../../stylesheets/mixins';
+@use '../../stylesheets/mixins' as *;
 
 .selectionmap-leaflet-container {
   height: 100%;