Add variable for inputs border width

Signed-off-by: Marco Ambrosini <marcoambrosini@proton.me>

Author: marcoambrosini

core/css/variables.scss

@@ -90,6 +90,8 @@ $color-border: nc-darken($color-main-background, 7%) !default;
 // darker border like inputs or very visible elements
 $color-border-dark: nc-darken($color-main-background, 14%) !default;
 
+// Border width for input elements such as text fields and selects
+$input-border-width: 1px !default;
 $border-radius: 3px !default;
 $border-radius-large: 10px !default;
 // Pill-style button, value is large so big buttons also have correct roundness