Variable.js file from Native Base Customizer doesn't change Header Background Color

## node, npm, react-native, react and native-base version
```
"native-base": "^2.7.0",
"react": "16.2.0",
"react-native": "0.54.0-rc.3",
```
## Expected behaviour
Variable.js file from the customizer changes the Header background color

## Actual behaviour
Doesn't change the Header background color (but for Android it changes the color of the bar above the header)

## Steps to reproduce should include code snippet and screenshot
Use the Native Base customizer to change the header color for the application and download the variable.js file. Attempt to apply it using a StyleProvider.

For me, the variable.js file set the color in this format: `"toolbarDefaultBg":"rgba(254,105,23,1)"`
For some reason, React Native didn't recognize this, so the default blue was used. When I manually change it to this: `"toolbarDefaultBg":"#FE6917"` then it works like intended.

## Is the bug present in both iOS and Android or in any one of them?
Present for android, haven't tested on iOS

## Any other additional info which would help us debug the issue quicker.
Nope

Start by tracing how the Native Base Customizer generates variable.js, focusing on the toolbarDefaultBg value and how StyleProvider applies it on Android. Reproduce the issue with the reported rgba value, compare it with the working hexadecimal value, and verify that the generated header background matches the configured color.

Written by the indexing model from the issue text.