Polish transactions page

[ErnWong]

User Story
As a user
I want a polished split bill user interface
So I don't mistake Sp/it with a scam app or a malware-loaded phishing site.
(as well as so that I feel more comfortable using Sp/it and get the impression that Sp/it is not buggy)

Priority
[]: High
[]: Medium
[x]: Low

Describe the solution you want to implement
Using the HiFi prototypes as the source of inspiration:

Compared with what we have at the moment:

In particular:

• Use our theme's pink/red heading colour
• Reduce size of heading
• Align heading with list item text
• Use theme's slight blue tint for text
• Highlight the current Transactions page on the sidebar with a white background.
• TODO: See below about new mockups.

The hifi-prototype did not take account of the nesting structure of the bills. New mockups will be needed to accommodate it. Those will probably be needed before this issue is worked on. #134

Describe alternatives you've considered
TODO

Issue Dependencies
Depends on #134

Additional context
TODO