packages/theme: make BackstageTheme a type and update usages

plugins/lighthouse/src/components/CategoryTrendline/index.tsx

@@ -18,10 +18,7 @@ import { Sparklines, SparklinesLine, SparklinesProps } from 'react-sparklines';
 import { useTheme } from '@material-ui/core';
 import { BackstageTheme } from '@backstage/theme';
 
-function color(
-  data: number[],
-  theme: typeof BackstageTheme,
-): string | undefined {
+function color(data: number[], theme: BackstageTheme): string | undefined {
   const lastNum = data[data.length - 1];
   if (!lastNum) return undefined;
   if (lastNum >= 0.9) return theme.palette.status.ok;
@@ -30,7 +27,7 @@ function color(
 }
 
 const CategoryTrendline: FC<SparklinesProps & { title?: string }> = props => {
-  const theme = useTheme<typeof BackstageTheme>();
+  const theme = useTheme<BackstageTheme>();
 
   if (!props.data) return null;
   return (