[REF] web: use $input-bg for o_inputs

Partially remove visual discrepancies between odoo and Bootstrap fields,
allowing to use BS variables to control fields' design across versions
and color-schemes.

This commit is preparatory to the introduction of dark-mode.

task-2710677

Part-of: odoo/odoo#102868

addons/web/static/src/legacy/scss/fields.scss

@@ -35,7 +35,7 @@
     border:$input-border-width solid var(--o-input-border-color);
     border-width: 0 0 $input-border-width 0;
     padding: $o-input-padding-y $o-input-padding-x;
-    background-color: var(--o-input-background-color, unset);
+    background-color: var(--o-input-background-color, #{$input-bg});
 
     // -- Nested o_input(s)
     .o_input {

addons/web/static/src/scss/bootstrap_overridden.scss

@@ -251,7 +251,7 @@ $kbd-bg: $o-gray-100 !default;
 $kbd-box-shadow: 0px 1px 1px rgba($o-black, 0.2), inset 0px -1px 1px 1px rgba($o-gray-200, 0.8), inset 0px 2px 0px 0px rgba($o-white, 0.8) !default;
 
 // Input
-$input-bg: $o-white !default;
+$input-bg: $o-view-background-color !default;
 $input-focus-border-color: $o-brand-primary !default;
 $form-check-input-checked-color: $o-brand-lightsecondary !default;
 $form-check-input-checked-border-color: $o-brand-primary !default;

addons/web/static/src/search/search_panel/search_view.scss

@@ -69,6 +69,7 @@
         width: 75px;
         border: none;
         outline: none;
+        background-color: var(--o-input-background-color, #{$input-bg});
     }
 
     .o_searchview_autocomplete {
@@ -193,6 +194,7 @@
                 border-bottom: 1px solid $o-brand-secondary;
                 margin-bottom: 15px;
                 width: 100%;
+                background-color: var(--o-input-background-color, #{$input-bg});
             }
     
             .o_searchview_facet {