doc: css: fix sphinx tabs style issues

Flag Sphinx tabs style properties as !important, it looks like the
default style used by the latest extension version take precedence.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
diff --git a/doc/_static/css/custom.css b/doc/_static/css/custom.css
index 2e14bfa..1f69b3d 100644
--- a/doc/_static/css/custom.css
+++ b/doc/_static/css/custom.css
@@ -393,16 +393,16 @@
 .ui.tabular.menu .active.item,
 .ui.segment,
 .sphinx-tabs-panel {
-    background-color: var(--code-background-color);
+    background-color: var(--code-background-color) !important;
 }
 
 .sphinx-tabs-tab {
-    color: var(--link-color);
+    color: var(--link-color) !important;
 }
 
 .sphinx-tabs-tab[aria-selected="true"] {
-    background-color: var(--code-background-color);
-    border-bottom: 1px solid var(--code-background-color);
+    background-color: var(--code-background-color) !important;
+    border-bottom: 1px solid var(--code-background-color) !important;
 }
 
 /* Code literals */