Update use of Sass variable background variant colors

The hardcoded Sass variable assignments to lighter variant background colors
does not work well with the theme-color functions that are used.

Use Bootstrap's built in theme-color-level function to programatically get
a lighter variant color to use for component backgrounds.

https://getbootstrap.com/docs/4.5/getting-started/theming/#functions

Signed-off-by: Yoshie Muranaka <yoshiemuranaka@gmail.com>
Change-Id: I9d5a1a66c92d347ba1797be5e0810a24c34094f9
diff --git a/src/assets/styles/bmc/helpers/_functions.scss b/src/assets/styles/bmc/helpers/_functions.scss
new file mode 100644
index 0000000..456caf3
--- /dev/null
+++ b/src/assets/styles/bmc/helpers/_functions.scss
@@ -0,0 +1,5 @@
+// This function is usually used to get a lighter
+// theme variant color to use as a background color
+@function theme-color-light($variant) {
+  @return theme-color-level($variant, -11.3);
+}
\ No newline at end of file