Tweak the orange button color (#8266)

Updates the orange button's color such that its color contrast is at least 3:1 on both our body's and header's background colors. Currently, the color is dark enough on our white background, but too dark for our blue background.

Addresses #8265

Author: loic-sharma

src/Bootstrap/dist/css/bootstrap.css

@@ -166,7 +166,7 @@
 @btn-info-border:                darken(@btn-info-bg, 5%);
 
 @btn-warning-color:              #fff;
-@btn-warning-bg:                 darken(@brand-warning, 5.75%);
+@btn-warning-bg:                 darken(@brand-warning, 4.5%);
 @btn-warning-border:             darken(@btn-warning-bg, 5%);
 
 @btn-danger-color:               #fff;