fix(messageSpace): update color of scroll down button

[kaseyvee]

Changes

• update color of scroll down button

Addresses issue #156.

Will have to look into the how rusticDarkTheme aligns with MUI default styles for buttons with both variant='contained' and color='primary' .

Screenshots

Before

After

[kaseyvee]

@RenataDzotova @Laurendragonscale

[Laurendragonscale]

This looks good, I should get to adding the dark theme buttons to the library this week hopefully.