fix button font colors

resources/assets/themes/theme7.less

@@ -188,11 +188,11 @@ THE SOFTWARE.
 @btn-info-bg:                    @brand-info;
 @btn-info-border:                darken(@btn-default-bg, 10%);
 
-@btn-warning-color:              @btn-default-color;
+@btn-warning-color:              @gray-darker;
 @btn-warning-bg:                 @brand-warning;
 @btn-warning-border:             darken(@btn-default-bg, 10%);
 
-@btn-danger-color:               @btn-default-color;
+@btn-danger-color:               @gray-darker;
 @btn-danger-bg:                  @brand-danger;
 @btn-danger-border:              darken(@btn-default-bg, 10%);